When Prince Harry and his girlfriend Meghan Markle kissed at the polo last week, their public clinch led to fevered speculation that an engagement announcement could be imminent.
As they approach their one-year anniversary, senior staff at the palace think news of an impending wedding could be made "fairly soon", certainly before the end of the year.
Well-placed sources say the relationship is extremely serious, and that Meghan's last‑minute invitation to Pippa Middleton's wedding is the ultimate endorsement, painting her as a potential princess.
Those close to the couple say they are simply waiting until the time is right. August 4 - Meghan's birthday, and that of the late Queen Mother - has been tipped as a day to watch.
So is it time for the Queen to buy a new hat? We tot up the tell-tale signs that Harry and Meghan are set to get engaged.
MEGHAN'S CAREER REBRAND
Saucy maids, raunchy nurses, a cage fighter, a princess desperately seeking her prince... Meghan has starred in more than a few box-office flops.
For six years she's played Rachel Zane in American legal drama Suits, a role that catapulted her to fame and - along with her royal romance - made her 'the most Googled actress in the world'.
But since her relationship with Harry came to light, Meghan's acting career has gone quiet.
And in the past few months it seems she's deliberately withdrawn from the spotlight, focusing instead on her charity work and rebranding herself as a humanitarian campaigner in the mould of Angelina Jolie.
In an article she wrote for Time magazine in March, entitled How Periods Affect Potential, she was described as an 'actress and humanitarian'.
Meghan's two-year contract with Suits comes to an end in 2018, and sources say she has no intention of staying beyond that.
Her name hasn't been associated with any films since last year. Hollywood insiders say her agents are demanding higher acting and appearance fees, with some clients offering £77,000-plus for Meghan to host speaking engagements or product launches.
But she's getting increasingly picky about what she does.
Last month, the actress quit her lucrative role as ambassador for Canadian fashion brand Reitmans, which saw her design a range of sexy, skin-tight clothing including a vegan leather skirt and bodysuit.
The decision is said to have left the chain "baffled and somewhat annoyed", given it only launched in November.
Harry is believed to be taking Meghan to Lesotho, the tiny land-locked country in South Africa where he founded his children's charity Sentebale, in the autumn.
The trip will mark a new milestone in their relationship, not least because it would be frowned upon to jet off on a taxpayer-funded trip with a mere girlfriend in tow.
(Harry learned his lesson in 2010, when he spent five weeks in the African wilderness with then girlfriend Chelsy Davy. Although the trip was private and paid for by Harry, he was criticised for using royal bodyguards.)
Sources say this is almost certain proof Harry and Meghan plan to be engaged by then.
Africa is, after all, where William proposed to Kate, getting down on one knee at the foothills of Mount Kenya, in October 2010.
Harry and Meghan are thought to be spending a few nights at the stunning Maliba Mountain Lodge, Lesotho's only five-star hotel, where a stone-and-thatch chalet costs from £230 a day.
Each has its own private veranda, open fireplace and a giant bath, and guests can enjoy hiking in the Maluti Mountains, pony trekking and spa treatments. Evenings are spent in the lodge's gourmet restaurant, where they can dine on cider-roasted pork belly and molten chocolate pudding, while sipping on a 2012 Pinotage aptly made "for princes and paupers, jesters and kings".
Harry will be keen to introduce Meghan to his chums in the country, notably Prince Seesio, his charity co-founder, whose palatial residence may form the couple's base. Other potential pit-stops include Botswana, Zambia and the Ulusaba Game Reserve in South Africa, owned by Virgin tycoon Richard Branson.
MEETING THE FAMILY
As an American divorcee (Meghan was briefly married to TV producer Trevor Engelson), she has to work hard to escape the shadow cast by US socialite Wallis Simpson, whose 1936 liaison with Edward VIII and his subsequent abdication plunged the Royal Family into crisis.
Though she hasn't yet been introduced to the Queen, Meghan has met several of Harry's relatives, including Charles, William and Kate and their children (whom she's seen at Kensington Palace at least twice) and cousin Eugenie, with whom she is said to be "very friendly".
Harry values his sensible sister-in-law's opinions, and though Kate isn't going out of her way to befriend Meghan - insiders say her mind is simply focused on her sister Pippa's wedding - once that's over, she'll make more of an effort.
The Queen's point of view, too, is incredibly important. As well as seeking her counsel, Harry will want her blessing.
Courtiers believe Harry's closeness to his grandmother means Meghan will be introduced to Her Majesty at Windsor Castle, possibly at an informal lunch attended by Prince Philip, the Cambridges and the Wessexes - Sophie and Edward.
As for Meghan's family, Harry has met both her parents (who are divorced) on separate trips to Toronto last June and July.
He has an easy, chatty relationship with her father Thomas, but her protective mother Doria is more guarded in her appraisal of the charismatic prince.
SOCIAL MEDIA SILENCE
For someone with 1.6 million followers on Instagram and another 374,000 fans on Twitter, Meghan's social media accounts have been conspicuously silent of late. Last November the actress was updating her pages every few days, with colourful photos of food, sultry modelling shots and images of her two dogs, Guy and Bogart. The sort of trivial fluff celebrities normally post.
This year, however, Meghan has posted just four new pictures and tweeted seven times, each one focusing on virtuous causes such as International Women's Day and her charity work.
It's not difficult to imagine Palace staff urging her to tone down her online presence - so unseemly for a princess.
Tellingly, last month Meghan also shut The Tig, the lifestyle website she founded in 2014, and changed the settings so none of her old posts, on topics such as food and drink, music and travel, can be seen.
A source insisted the decision was all about prioritising - "The Tig has been a labour of love for Meghan, but it's a full-time job" - but some fans read a hint of reluctance in her farewell note.
"After close to three beautiful years on this adventure with you, it's time to say goodbye," she wrote.
Meghan's transformation into princess-in-waiting extends to her wardrobe, which has recently been overrun by expensive designer brands befitting a royal.
Gone are the vest tops, sports leggings and leather jackets she favoured in the early days of her courtship; now the glare of the spotlight is upon her, she has to look the part.
Elegant day-dresses by Burberry and Prada, Mulberry and Goyard handbags and shoes by Chanel and Jimmy Choo have all become staples. Even her dressed-down look has had a makeover, with Stella McCartney trainers and Barbour jackets.
At the polo last week, she sported a navy sleeveless dress by chic British designer Antonio Berardi (the £1475 dress has now sold out in all sizes), and the following day donned a Burberry silk trench worth £1709.
A source close to the actress says Meghan 'will always defy tradition as she likes edgy designer clothes with a twist'. But the above seems remarkably tame.
As a sign of her growing style credentials (and the influence she, like the Duchess of Cambridge, wields), fans have now set up a dedicated fashion website, Meghan's Mirror, which lists everything she's ever worn. Her Kate-esque bouncy blowdry shows Meghan's hair is also getting the royal treatment, while even her make-up has taken a turn for pricier, princess-appropriate brands such as YSL, Dior and Charlotte Tilbury.
Their matching £2.50 beaded bracelets, bought by Harry in South Africa to support the charity Relate, gave the game away after the couple were both spotted wearing them in October.
But the pair, who reportedly love token jewellery, have shared plenty of other romantic trinkets that show how serious they are.
In December, Meghan sported a £230, 14-carat gold necklace by Maya Brenner, customised with 'M' and 'H' charms, believed to be a gift from her beau.
The rumour mill went into overdrive when she was spotted wearing a 'promise ring' - a plain gold band worn on the right-hand ring finger - on an outing in Toronto this year, alongside what appeared to be a ring emblazoned with an 'H' (later revealed to be a £59 Missona double arrow ring).
At the polo, Meghan's hands had none of her customary gems. Is she clearing the way for a more significant sparkler?
Sources say Meghan's security in the US and Canada has recently been upped, ensuring a bodyguard is on hand every time she leaves the house. They're private security staff paid for by Meghan herself.
Royal protection officers are only on duty when she's out and about with Harry.
The pair sometimes fly by private jet (though Harry is known to nip on an economy flight), and when she's in the UK, Harry's PA, Clara Madden, books Meghan a chauffeured car.
At the Palace, guards have apparently taken to waving the actress through the gates without the same screening as other guests - the royal equivalent of giving your girlfriend a key to your flat.