Boris Johnson has been called “cowardly” for refusing to comment on Donald Trump’s false claim that he has won the US presidential election. 

Speaking in the White House in the early hours of Wednesday, Trump claimed without evidence there had been “fraud” in the electoral system, and said: “We did win this election.

“We want the law to be used in a proper manner. So we will be going to the US Supreme Court. We want all voting to stop.”

It stood in stark contrast to the reaction of his rival Joe Biden, who called for all votes to be counted and for Americans to “be patient” as the race continued.

The election hangs in the balance, with many swing states, such as Pennsylvania, yet to declare a result, 

The prime minister was challenged in the Commons by Labour leader Keir Starmer to criticise the president amid mounting anger over the speech, but he refused. 

But Johnson told MPs: “We don’t comment as a UK government on the democratic processes of our friends and allies.”

He had said: “Whatever the results, would the prime minister join me in saying it’s not for a candidate to decide which votes do and don’t count or when to stop counting.

“The next president must be the free and fair choice of the American people.” 

Deputy Labour leader Angela Rayner tweeted strong criticism of the decision by Johnson, saying: “The prime minister refused to even say that all votes in the US election should count.

“That is one of the most cowardly statements I’ve ever heard.”

Foreign secretary Dominic Raab had also refused to offer a view earlier on Wednesday, saying he would “not comment on the commentary” around the election – but a number of senior backbenchers have spoken out. 

Tory former health secretary Jeremy Hunt tweeted: “Dear American friends… The reputation of democracy is at stake and the world is watching. Please proceed carefully.” 

Tory MP and former trade secretary Liam Fox said both presidential candidates were engaging in “political positioning” but said of Trump’s stance: “You can’t claim to have won when some of the bigger electoral votes have still to come.” 

Noting the key states of Michigan, Arizona and Pennsylvania were at that time yet to record a result, Fox added: “I think it is wishful thinking, but you can’t claim that you have won until all the votes have been counted.”