Sir Sadiq Khan shared words of advice for New York mayoral hopeful Zohran Mamdani after his triumph in the Democratic primary.
Mamdani beat former New York Governor Andrew Cuomo last month following a successful campaign.
Since the victory, the could-be Mayor and Khan have spoken over the phone, a source told The Telegraph.
Khan was last year re-elected as London mayor for a historic third term, with a winning margin of 275,000 votes.
The call was "warm and collegial and Mayor Khan had some words of advice”, the unnamed source said.
The pair allegedly did not discuss strategy, however.
A source close to Mamdani’s campaign told The Telegraph Khan has a “unique set of experiences” being the only Muslim mayor of a Western city, which Mamdani will likely talk with the Labour mayor about if he is elected.
Conservative London Assembly Member Susan Hall however criticised Khan over the alleged phone call, saying he "desperately wants" the approval of the overseas politician.
"If Khan was more inclined to pick up the phone to his colleagues in Westminster rather than politicians across the sea whose approval he desperately wants, London might actually have been better off at the last spending review, the Harrow councillor said.
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|Sir Sadiq Khan is said to have shared advice with Zohran Mamdani after his primary win
"Still, if Mamdani wants advice on how to be petty and evasive he's certainly got one of the best advisors on the planet."
Following his primary win, Mamdani pledged to "shift the tax burden" on "richer, whiter neighbourhoods" if he is to become mayor in November's election.
He last month released a memo entitled “supporting homeowners and ending deed theft".
In the housing policy document, Mamdani said: “The property tax system is unbalanced because assessment levels are artificially capped, so homeowners in expensive neighbourhoods pay less than their fair share."
The Democrat has previously described capitalism as “theft” and endorsed controversial plans to “defund the police” following Black Lives Matter protests.
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The president of Partnership for New York, Kathryn Wylde, which represents businesses in the city, said industry leaders were “terrified” by the Democratic candidate’s rise.
Mamdani’s policies also include introducing city-owned grocery stores, universal free childcare and heavy tax rises on all residents earning more than $1million a year.
The mayoral candidate has already clashed with Republican US President, Donald Trump, who said he would "have to arrest" Mamdani if he stopped the Immigration and Customs Enforcement's (ICE) from arresting non-American citizens who commit serious crimes, dubbed "criminal aliens".
In response, Mamdani posted to Instagram on July 1: "The President of the United States just threatened to have me arrested, stripped of my citizenship, put in a detention camp and deported.
"Not because I have broken any law but because I will refuse to let ICE terrorise our city.
"His statements don't just represent an attack on our democracy but an attempt to send a message to every New Yorker who refuses to hide in the shadows: if you speak up, they will come for you.
"We will not accept this intimidation."
The President has also previously labelled Mamdani a "communist lunatic".