• Ronan O'Connell is an Australian traveller who is now based in Asia, and has visited more than 50 countries for his travel writing and photography work. You can follow his adventures on Twitter.
From assassination sites to secret meeting spots, hidden intelligence headquarters and streetlights used to hide coded notes, London is littered with places linked to the long-running spy wars between the UK and Russia.
This intelligence battle is back in the spotlight after UK authorities this month accused two alleged Russian spies of poisoning a former Russian double agent and his daughter in the UK, reports news.com.au.
Sergei Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench in the city of Salisbury in March after what UK authorities say was an attempted murder by two Russian men who immediately fled England.
The accused men have denied any involvement, while the poisoned pair have since recovered.
Espionage has long been common in the UK, particularly in London, which has been a key location for spying.
The fascinating history of these spy wars is revealed at the following London sites, which I visited in the wake of this latest incident.
THE SPY LIGHT POLE
At first I passed by without even noticing it, which is just what the Russian KGB would have wanted. In Mayfair, one of the ritziest parts of London, there's a lamp post which looks just like any other.
But if you stop to inspect this black post on Audley Square you will see that it has a small hinged door at about hip height.
For many years undercover KGB agents reportedly received coded notes hidden inside this compartment. Known as "dead letter drops", such sites were where agents could go to receive instructions on their latest mission from their superiors.
When a note had been placed inside the post, the KGB would leave a chalk mark on its exterior to alert their operative.
The UK's intelligence agencies, MI5 and MI6, were unaware of this covert use of the lamp post until learning of it after Russian spy Oleg Gordievsky defected to the UK in the 1970s.
On the sunny afternoon when I visit, several tables of people are eating and drinking outside the Millennium Hotel in Knightsbridge. Some of them likely are unaware that it was here where a former KGB operative is believed to have been fatally poisoned in 2006.
Alexander Litvinenko, who was living in exile in the UK after leaving the KGB, was at the hotel with a group of fellow ex-KGB agents when it is suspected the radioactive isotope Polonium-210 was slipped into his tea.
Litvinenko, 43, soon became violently ill and died three weeks later in an incident which bears eerie similarities to the attempted assassinations this year of Sergei Skripal and his daughter Yulia.
SPY MEETING PLACES
Hiding in plain sight is a common tactic for spies, and this extends to their meeting places. As I stand on the Blue Bridge in St James's Park, amid a torrent of tourists taking photos with Buckingham Palace in the background, I imagine all the very public yet extremely secretive conversations which occurred here between spies.
This bridge for a long time was used by British MI5 and MI6 agents to meet informants or other intelligence operatives.
Amid the crowds they would exchange confidential information in a casual manner, giving the impression they were merely friends having a chat. I scan the crowd for any people engaged in a particularly deep looking conversation but spot none.
I do the same when I pull up a stool half an hour later inside the Red Lion, an unassuming British pub which just happened to play a major role in the Britain v Russia spy wars. It was here in the 1960s that the KGB managed to turn Frank Bossard, a British spy.
A heavy drinker who had serious gambling debts, Bossard was a regular at the Red Lion, where he struck up a friendship with a Russian spy based on their mutual interest in coin collecting.
Having gained Bossard's trust, the KGB agent convinced him to leak highly sensitive information on British military radar and missile systems, giving the Russians a major intelligence victory.
BUILDINGS WITH HIDDEN SPY PURPOSES
Buildings across London have been secretly used at various stages by Britain's intelligence agencies, whether as bases or as recruitment spots. London has long been famous for its private members' clubs, where politicians, wealthy professionals and successful business people meet to network or just relax with a cocktail.
Some of these clubs were reportedly used by MI5 and MI6 as prime recruiting grounds, including three clubs which are within a few minutes' walk of the Red Lion.
The In and Out Club, Boodle's Gentlemen's Club, and White's Gentlemen's Club are swanky venues which grant access only to their small list of well-connected members. As such I can only admire them from the foot path, watching men in tailored business suits come and go. It was in their salubrious interiors that British intelligence agents would sound out potential recruits, assessing their smarts and suitability to the spy game over a glass of whiskey.
Slightly more than a kilometre south of this cluster of clubs I find two buildings which were once covert bases for British intelligence. One of them, now known simply as 54 Broadway, was the headquarters of MI6 for almost 40 years up until 1964.
Initially MI6 tried to conceal their presence, registering themselves as the Minimax Fire Extinguisher Company, but over the years their location became well-known.
Slightly more than 100 metres from there I pause outside the up-market St Ermin's Hotel, which was a key base for MI6 and other British intelligence units during WWII. Many key spy missions during that war were hatched and launched from this building, where tourists now rest their heads, blissfully unaware of the building's shady history.
The same tourists could spend a day walking around central London without realising they had passed half a dozen key locations in Europe's ongoing spy wars.