Britain’s longest-serving Labour Mayor is facing ‘deselection’ after local party members voted against him being automatically chosen as their candidate.

In fresh evidence of the new mood among Jeremy Corbyn’s grassroots, Sir Robin Wales will now have to take part in an open contest if he wants to continue his 23-year rule as leader of Newham Council in East London after the May 3 local elections.

Sir Robin is facing a strong challenge from local councillor Rokhsana Fiaz, who is backed by Momentum activists but also by others from all wings of the party who want a change at the helm of the borough that hosted the 2012 Olympics.

The directly elected Mayor is seeking an unprecedented fifth term in an area of London that is dominated by Labour, which has 59 out of the borough’s 60 councillors and where a lone independent councillor provides opposition.

Sir Robin, a veteran centrist who once battled against Militant in the 1970s and 1980s, narrowly won a ‘trigger ballot’ in 2016, but the process had been rescinded after a legal challenge and claims of irregularities.

This weekend, all 20 local Labour wards in the two local parties in Newham voted unanimously for an open selection and on Tuesday the London Labour Party opened applications for the candidacy.

With nearby Haringey Council leader Claire Kober quitting over objections to her public-private housing plans, the prospect of the removal of another senior Labour town hall chief underlines a big shift since Corbyn pledged more local democracy under his leadership.

An online-only ballot of Newham members will take place and the winner is expected to be unveiled within a month, sources told HuffPost UK. A shortlist of candidates is expected on Tuesday.

The London Labour Party later confirmed the race will be run between March 1 and March 16. That will give the candidate just a few weeks to campaign before the May 3 election is held.

The one-member, one-vote election – which will allow Momentum and other groups locally to mobilise their members – could offer challenger Fiaz the chance to take the candidacy.

However, the London party faces possible fresh criticism after it emerged that newer members will not be allowed to vote. The ballot is open only to members who joined before July 1, 2016 – the ‘freeze’ date for the previous ‘trigger ballot’ contest.

Fiaz, who was born and raised locally, tweeted that she hoped to now win the selection contest.

She told HuffPost: “In the coming weeks I will be setting out my vision for Newham on housing, jobs, education and health; and where residents are at the heart of everything we do. 

“I’ll be starting that now by listening to members and residents about how we can build Newham’s future together so everyone has the opportunity to succeed and thrive.”

She has swiftly won support from Momentum-backed members of the party’s Conference Arrangements Committee, who are among those who believe BAME and women candidates have been under-represented.

Newham elected Labour’s first directly elected Mayor in 2002. But the party has faced criticism as its big ‘Metro Mayors’ – Sadiq Khan in London, Andy Burnham in Manchester and Steve Rotheram in Liverpool – have all been men.

Fiaz is also expected to make a key part of her manifesto a referendum on the very existence of the directly-elected mayor model devised by Tony Blair, with some local members believing it gives too much power to one individual.

Wales has the strong support of the GMB union in Newham, whose branches were the only ones to oppose an open contest.

Wales told HuffPost: “I will stand proudly on my record, whether that’s private-rented sector licensing, universal free school meals, and my radical vision to stand up against austerity and for social justice in the future.

“That means working to fix the Tory housing crisis, securing council services, improving skills and education, making our streets safer and creating a greener, healthier borough.”

In December, Wales attacked the legal challenge against the trigger ballot process, declaring “it is deeply regrettable that at this late stage in our preparation, and with local elections just a few months away, we have to appease a minority for the sake of their own gain”.

But he surprised his critics a fortnight ago by agreeing to an open selection.

Newham Momentum said he had “wasted time” by changing his mind at such a late stage.

Newham has seen a huge increase in Labour party membership since Corbyn became leader, from just 900 to more than 3,000 today.

Wales points to his record as mayor, including freezing council tax for nine years, creating 35,000 jobs through Newham’s Workplace scheme and the Every Child initiative.

In his youth, he was reportedly one of the Scottish Labour Students who famously travelled in the ‘Ice Pick Express’, a coachload of moderates named after the weapon used to kill Leon Trotsky.

As detailed by journalist Michael Crick, the coach travelled to a student conference to ensure Militant students were outvoted in 1976.

A local Momentum source told HuffPost that Fiaz had impressed members with her backing for some of the movement’s ideas, while also retaining ‘broad’ soft left positions on other issues.