Scotland Yard officers could be forced to live in Canada for months at a time, it has emerged, amid a row over whether the Duke and Duchess of Sussex should contribute towards an estimated £10 million ($20.5m) annual security bill.
The pair's protection arrangements will create a huge strain on police resources now Canada has confirmed it will stop guarding them when they step down as working royals. They face a fierce public backlash if seen to be costing the British taxpayer millions while raking in a private fortune.
Royal sources have suggested the couple will contribute towards their own security if they are commercially successful.
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They are understood to be intending to reimburse the taxpayer for security provided on private business engagements with no royal connection.
However, police sources admit they still have no idea how the security plan will work, precisely where the couple and their son, Archie, will live and who will foot the hefty security bill.
A Scotland Yard source said: "This situation is completely unprecedented in the modern era and there are still many details that need to be worked out before any long-term decisions can be made. Negotiations are ongoing."
Security experts highlighted various scenarios but all agreed the threat against Harry was credible, because of the "double whammy" of his royal lineage combined with having served in Afghanistan.
The level of protection the family is afforded will ultimately be decided by the Royal and VIP Executive Committee, chaired by former civil servant Sir Richard Mottram, based on an assessment of the impact if they died, the level of threat, risks and vulnerability to assassination attempts.
The committee has been meeting regularly in a bid to hammer out arrangements.
However, without a clear idea of the couple's future plans, where they will live and how much they plan to travel, discussions can only go so far.
The Government is deemed to have a duty of care towards the royal family and although the Met may provide close protection, the Government could also be asked to contribute towards the cost. One police source said: "The team is likely to consist of three or four Met officers who will be based full-time wherever the Duke and Duchess are in the world. The Met will probably ask for volunteers."
The Sussexes' decision to step away from public duties and move abroad to earn their own money has prompted demands from some quarters that they pay for their own security.
However, Dai Davies, former head of the Met's royal protection squad, said the threat against the couple was too severe for them to hire a private security firm. "Several individuals have threatened to kill Harry in the past few years," he said. "There is a credible threat. Access to intelligence and risk assessment are absolutely essential. They will also have to guarantee that their personal protection team is adequately trained. The only way this is guaranteed is to use specially trained officers."
He acknowledged that the predicament facing the royal protection squad was unprecedented.
"There is no formal mechanism in place for this kind of arrangement," he added. "I've never heard of external forces being paid privately by the Met but it could happen."
Davies admitted he was surprised the issue had been allowed to become a matter of public discussion.
"There has clearly been a breakdown in communications at every level," he added. "This scenario should not be a matter of public debate. It's an indictment on all concerned.
Simon Morgan, another former royal protection officer who now runs his own private consultancy, said the final package would be commensurate with the role the Sussexes decide to take on the global stage.
"If they come off the Internationally Protected Persons list, they become ultra-high net worth individuals, many of whom, such as Bill Gates and Sir Richard Branson, do have round-the-clock protection," he said.
"It's a strategic decision. But whether it's a private team or the Met, there will be one team who will look after their physical, cyber, technical security and intelligence. It will all be wrapped into one package.
"If they are earning money and are able to justify and pay for their protection team, then why would they not pay for it, like others do? It is different if their day-to-day role has any element of public duty.
He said Princess Beatrice and Princess Eugenie, whose private security teams funded by the Duke of York, proved that royals could employ private companies able to liaise with the Met about potential threats.