Donald Trump deplored Sadiq Khan as 'a national disgrace' just hours after reigniting their feud with a stinging lash as London saw three murders in 24 hours.
The US President tweeted, 'He is a national disgrace who is destroying the City of London!'
This followed a statement from the mayor on Saturday night, saying he would not 'waste his time' with Trump after he wrote: 'LONDON needs a new mayor ASAP. Khan is a disaster - will only get worse!'
Two teenagers were murdered within 12 minutes of each other, 12 miles apart in the capital on Friday, while a man was stabbed to death on Saturday afternoon - making three killings within just 24 hours.
Khan clashed with Trump when he jetted in for a state visit earlier this month, with the Mayor of London dubbing him a 'fascist' before Air Force One had even taken off.
On June 3, Trump rapidly hit back at Khan after his 'fascist' comments and branded the mayor a 'stone cold loser' in a two-part tweet as he touched down in Britain.
The Labour mayor then accused the President of 'playground behaviour' and claimed he was a 'poster boy' for the far-right.
Jeremy Corbyn said it was 'absolutely awful to see @realDonaldTrump using the tragedy of people being murdered to attack the Mayor.'
The Labour leader tweeted: '@SadiqKhan is rightly supporting the police to do their job while Katie Hopkins spreads hateful and divisive rhetoric. They seek to divide at a time we need to come together.'
Trump's ferocious tweet comes after an 18-year-old boy was stabbed to death in Wandsworth on Friday afternoon, minutes before another 19-year-old man was fatally shot in Plumstead.
Just hours later, a man in his 30s was killed in Tower Hamlets, east London.
Three men were also stabbed in a separate attack in Clapham during a weekend of violence in the capital.
The Metropolitan Police said 14 people - including several boys and a girl - have been arrested in connection with the incidents in Wandsworth, Plumstead and Clapham.
Retweeting a post by commentator Katie Hopkins about the killings, the President said the capital needed a new mayor 'ASAP'.
The original post by Hopkins shared two screenshots of BBC News articles detailing the violence.
Around five hours later, Trump fired off his 'national disgrace' tweet in response to American journalist Jessica Fletcher who revealed: 'Khan is the reason I don’t feel like visiting London anytime soon.'
In response to Donald Trump's original tweet, a spokesman for Sadiq Khan said the mayor was 'focusing on supporting London's communities' following the 24 hours of violence.
The spokesman said: 'Sadiq is focusing on supporting London's communities & over-stretched emergency services.
'He has been in regular touch with senior Met police officers last night and throughout the day.
'His thoughts are with the victims' families. He is not going to waste his time responding to this sort of tweet.'