Trump wrong to criticise Khan: May

British Prime Minister Theresa May says Donald Trump was wrong to criticise London Mayor Sadiq Khan in the wake of the London Bridge terror attack.

The US president, during a series of tweets following Saturday night's attack, challenged Khan for saying there was "no reason to be alarmed" when seeking to reassure Londoners.

May initially sidestepped questions about Trump by choosing to praise Khan's response.

Asked if Trump was wrong about his assessment of Khan, the prime minister told reporters on Monday: "I think Sadiq Khan is doing a good job and it's wrong to say anything else - he's doing a good job."

Trump on Sunday seized on comments by Khan, who said Britons should not be alarmed to see more police in the streets after three men drove a van into pedestrians on London Bridge before stabbing others nearby.

"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!'" Trump tweeted.

"We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse."

In response, a spokesperson for the London mayor said Khan was "busy working with the police, emergency services and the government to co-ordinate the response to this horrific and cowardly terrorist attack."