Theresa May has suggested Donald Trump was wrong to criticise Sadiq Khan and said the London Mayor was doing "an excellent job".
Following the weekend terror attack, the US president slammed Sadiq Khan for saying there was "no reason to be alarmed" when seeking to reassure Londoners.
However his tweet was based on a clear misinterpretation of some of the mayor's comments in the aftermath of the attack.
In an interview with the BBC, Mr Khan made it clear he was advising people not to be concerned by the presence of additional armed police officers on the streets in the days to come - and not about the attack itself.
This appeared to irk the US president, who later said his "excuse" was "pathetic", and that "MSM [mainstream media] is working hard to sell it."
Earlier the Prime Minister, taking questions after a central London election campaign speech, initially sidestepped questions on Mr Trump's reaction before praising the Labour mayor.
She told reporters: "I think Sadiq is doing an excellent job.
"We're working with the Mayor of London and City Hall to ensure, for example, that the transport network is able to get back up and running so that people can go about their business.
"I've been very clear, I've been very happy to say when I think President Trump is wrong - to have taken America out of the climate change agreement, the Paris agreement.
"The United Kingdom stays in it and we believe it's an important international agreement - so I'm not afraid to say when I think President Trump is getting things wrong."
Mr Khan told ITV News on Monday he "simply hasn't got time to respond to ill-informed tweets" from Donald Trump.
He said: "I've got to be honest since Saturday night my focus has been working closely with police, with the emergency services and with the Government to deal with the horrific attack."
However addressing police funding, the London mayor also suggested he does not welcome Mr Trump's planned state visit later this year.
He said London has other responsibilities due to its status as a capital city: "From major events, there's a big cricket match today, to state visits - some welcome, some less so.
"But the fact is we need to make sure the police have the resources and support they need, and as the mayor my job is to make our city safe and I'll do my best... to get the tools they need."