Tony Blair has rebuked Keir Starmer for his lack of support for Donald Trump's war on Iran, telling the Prime Minister: 'We should have backed America from the very beginning'.

Amid mounting diplomatic tensions between London and Washington over the conflict, Sir Tony warned his successor as Labour leader: 'If they are your ally and they are an indispensable cornerstone for your security... you had better show up'.

The former Prime Minister's dramatic intervention comes after President Trump described Sir Keir as 'not Winston Churchill' for initially denying him permission to launch strikes on Iran from UK territory, including the joint-US base on Diego Garcia in the Chagos Islands.

After Sir Keir relented, saying he would allow the US to fly missions for 'specific and limited defensive purposes', Trump said that he had been 'very disappointed' by his British counterpart.

Sir Tony's criticism of Sir Keir, at an event hosted by Jewish News on Friday, is likely to provoke anger in a Labour Party still scarred by his decision to join the American invasion of Iraq in 2003 on the false grounds that Saddam Hussein possessed Weapons of Mass Destruction (WMD).

Sir Keir justified his initial refusal to back Trump on the grounds that he did not believe in 'regime change from the skies'. The arguments were based on international law, but were driven by political calculations about the lack of an appetite in his Cabinet for emulating Sir Tony's unquestioning support for American military action.

A spokeswoman for Sir Tony said that his comments had been made at a private event and were not intended to be publicised. 

As US bombers landed in the UK this weekend ahead of a threatened 'surge' in strikes on Iran, Downing Street was on resignation watch for Cabinet ministers opposed to military action: it has been claimed that Energy Secretary Ed Miliband led a cabal of ministers who forced Sir Keir Starmer into withholding support for Trump at the start of the conflict a week ago. The four B-1 Lancers, which arrived at RAF Fairford in Gloucestershire on Friday and Saturday, are each capable of carrying 24 cruise missiles.

Sir Keir has since authorised the US to use UK airbases to bomb Iranian missile launch sites on the grounds of the protecting British interests and allies in the Gulf.

Sir Tony told the Jewish News event: 'I am not saying anything that I haven't already said to the government... I think we should have backed America from the very beginning'.

He added: 'We have got to be very clear about this as a country. We're depending on the American alliance for our country. They are not just an ally, they are an indispensable ally, right?

'Every single time you test an alliance you never test it when things are easy. You test it when it's hard. They were asking to use our bases to refuel... it's not like it was in Vietnam... not like the Iraq campaign where we had thousands of British troops.

'The American relationship matters. It matters particularly today. It's not a question of whether it's this president or that president. If they are your ally and they are an indispensable cornerstone for your security... you had better show up'.

Sir Tony then dismissed suggestions that unbending support for Trump would split the Labour Party, saying: 'People always complain....the problem for a leader is when you decide you divide... of course it's difficult. In the end, most of the MPs will know that going into the election it's going to be decided on different things'.

The former Prime Minister concluded: 'On foreign policy, I think people would just prefer you to be strong and out there and clear, even if they don't agree with you'.