Keir Starmer's just been given another reason to worry - and he's 3,000 miles away

The spectacular victory of Democratic socialist Zohran Mamdani in New York's mayoral election is yet another reason for Sir Keir Starmer to worry. New Yorkers jumped at the chance to vote for a charismatic Left-winger who pledged to make life more affordable by taxing the rich.

They put their faith in a 34-year-old who promised to deliver change in an era when the divide between the rich and the rest has opened into a chasm.

Sir Keir faces an epic electoral test next year when a raft of local elections will be held as well as the contests for the Welsh and Scottish Parliaments. This will be a referendum on Labour's record in power - and Sir Keir's challenge is to stop people deserting his weary party for more colourful alternatives.

Labour is tied with the Conservatives on 17%, just two points ahead of the Greens. Zack Polanski, the leader of the Green Party of England and Wales, is a passionate communicator and membership has soared on his watch. Left-leaning Britons who see Sir Keir's Labour as timid and part of the establishment which needs to be shattered now have a range of alternatives to vote for.

Jeremy Corbyn and Zarah Sultana's Your Party is offering a new home to Britons who want an unapologetic brand of socialism. The chaos surrounding the launch of their party does not mean it will not make an impact at the ballot booth; Mr Corbyn led a deeply divided Labour party but it won 40% of the vote in 2017 - more than Sir Keir netted last year.

Labour also has to fret about the Liberal Democrats under stunt-loving Sir Ed Davey. Memories of the party's role in the post-2010 Conservative-led coalition are fading and Sir Ed can present himself to centrist voters as a steady and experienced yet idealistic alternative to the PM.

Sir Keir might have hoped he could galvanise his base by saying a vote for Labour is the best way to stop the advance of Reform. But last month's Caerphilly by-election saw voters in the former Labour bastion swing behind Left-leaning Plaid Cymru to defeat Mr Farage's party.

Sir Keir will face calls from his own MPs to learn lessons from Mr Mamdani's populist triumph. The success of Lucy Powell in the deputy leadership contest demonstrated the desire for a Leftwards shift.

Mr Mamdani celebrated New York's multiculturalism, using his victory speech to salute "Yemeni Bodega owners and Mexican abuelas, Senegalese taxi drivers and Uzbek nurses, Trinidadian line cooks and Ethiopian aunties".

In contrast, Labour has spent its first year in power trying to respond to the surge in support for Reform UK while his Government has failed to stop small boat crossings. Sir Keir has said he regrets saying the UK could become an "island of strangers" in an immigration speech, but with Reform at 30% in the polls the PM ignores the rise of this ambitious party at his peril.

He can caution MPs that the Big Apple has a very different electorate to the UK as a whole, and that he needs to hold and win the centre-ground of British politics and prevent at all costs another bout of market turmoil. But this task will be even harder if the lesson delivered in May is that voters will turn out for Reform and the Greens and their radically different visions for change but not for Sir Keir's Labour.

If this happens, even more Labour MPs will have a New York state of mind and scan their ranks for Britain's answer to this barnstorming mayor.