The US ambassador to the UK has urged Britain to increase defence spending and push ahead with the F-35 programme.
Woody Johnson’s comments come amid suggestions of a black hole of at least £20 billion in the Ministry of Defence's budget over the next decade.
In a report published by the Public Accounts Committee last week, MPs warned that the MoD "simply does not have enough money to buy all the equipment it needs".
Talking to reporters about defence during a US Embassy housewarming reception, Mr Johnson said: "You're going to have to make trade-offs and go through the emotional and practical and philosophical arguments in terms of what you want to do, what you want to be, how important is defence, how you want to be perceived, by the US, but also by Russia and others.
"I know the president [Donald Trump] has stated that he doesn't want to pay for everything. He's going to rely on our allies….He wants everybody involved in mutual defence, not just the US doing everything."
Mr Johnson also described the F-35 Lightning programme, which Britain has a 25 per cent stake in, as "very, very interesting".
The programme has come under fire over capability and expense, with reports suggesting the effective cost of each jet is as much as £150 million when logistics and support are included.
On the affordability, Mr Johnson added: "To me, as an American, I would be looking at what do we want to be in the future, 10, 15, 20 years from now.
"Healthcare is always going to be an issue, education is always going to be an issue, transportation and infrastructure are always going to be issues, etc.
"But how important is it to defend yourself? I came over here ... my mission is security and prosperity, and you really can't have prosperity unless you have security."
American aviation giant Lockheed Martin is manufacturing the multimillion-pound supersonic aircraft, of which the UK has pledged to buy 138 over the coming decades.