The president of the TUC, a close ally of Jeremy Corbyn, has called for Labour to support a general election as soon as legislation halting a no-deal Brexit is imposed on the government.

Mark Serwotka also warned Labour MPs including Tom Watson to fall in line behind their leader amid an intense debate among senior party figures over the timing of a national vote.

The union leader’s comments come as Corbyn meets opposition leaders again to discuss when the parties may support Johnson’s call for an election.

Senior Labour figures, including Keir Starmer, have suggested that polling day should not take place until an extension to Brexit has actually been secured. The shadow chancellor, John McDonnell, has argued that Johnson cannot be trusted not to repeal the legislation against no deal.

Q&A How does the Fixed-term Parliaments Act work? Show Introduced by the Conservative-Liberal Democrat coalition, the act set in place five-year, fixed-term parliaments with elections to be held on the first Thursday in May in 2015, 2020, 2025 and so on.

In theory, the act makes the calling of any general election outside this term the decision of the House of Commons, not the prime minister. A vote by at least two-thirds of the Commons in favour of an early general election would formally fire the starting gun on the process – the input of the House of Lords is not required. The act also provides for an early poll if a majority of MPs pass a vote of no confidence in the government and no new government is formed within the following 14 days.

Some Labour MPs have said they do not want an election until after 31 October so that Johnson is forced to request an extension to article 50 or resign as prime minister.

Serwotka, who was due to address the first day of the TUC’s annual congress in Brighton on Sunday, said he favours going to the polls as soon as anti no-deal legislation is in place.

“My position is that we want an election, we want it as soon as possible once a no-deal Brexit is off the table. We have been demanding a general election and now we should get to it as quickly as we could,” he told the Guardian.

“I think that having an election leaving Boris Johnson any opportunity to push through a no-deal Brexit by default would be ridiculous. I think [Johnson’s supporters] know that people aren’t going to fall for their trap. It would be outrageous if we slipped out of the EU while holding an election,” he said.

Serwotka, the general secretary of the PCS public service union since 2001, said the current political crisis should mean that Labour’s deputy leader and others no longer “indulge” in plots or undermining tactics against Corbyn. “I think the dangers of perceived splits in the Labour party are damaging.

“The actions of some of the parliamentary Labour party such as Tom Watson and others have been really unacceptable. They must know as much as anyone else what is at stake. And I think there will be a demand to keep our eye on the ball here. Now is not the time to indulge in undermining Jeremy.”

Watson has been criticised by Corbyn supporters for calling for greater cooperation with the Liberal Democrats to stop Brexit and for allegedly “exacerbating fears” over antisemitism within the party – claims that Watson’s allies deny.

Serwotka, who rejoined Labour under Corbyn’s leadership, dismissed the idea of a national unity government led by a parliamentary grandee, adding that he would only support a Corbyn-led government.

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Be a British, Irish or qualifying Commonwealth citizen.

Be resident at an address in the UK (or a British citizen living abroad who has been registered to vote in the UK in the last 15 years).

Not be legally excluded from voting. You also need to be on the electoral register. You only have to register once, but will need to re-register if you have changed your address, name or nationality. The forms can be completed online. You may need details of your national insurance number and your passport if you’re a British citizen living abroad, and want to vote in England, Scotland or Wales. If you need help, you should contact your local Electoral Registration Office. You can use this service to find the address if you live in England, Scotland or Wales. If you live in Northern Ireland you need to contact the Electoral Office for Northern Ireland (EONI).

“I think the last hurrah [of Corbyn’s critics] is this silly notion that he could not be a caretaker prime minister so it would have to be Ken Clarke or Harriet Harman. Any Labour politician who gave that any succour is a disgrace,” he said.

The 151st TUC Congress will be dominated by the current political turmoil, Serwotka said, and will focus minds on how to get a no-deal Brexit off the table and then move towards a Labour government. “The obsession with Brexit has meant that so many issues have not got the attention of government that they should.

“While the unions want to talk about jobs, skills, pensions, wages, the starving of resources from the public sector, the starving of resources across the car industry, the government has not been on the ball,” he said.

During his final speech as president, Serwotka will call for the unions to embrace immigration and fight against a rise in racism. Johnson has been accused of stoking division and extremist views by suggesting last year that Muslim women in burqas looked like letterboxes or bank robbers.

“When political leaders in the US and the UK openly make racist remarks, it stokes up violence and intimidation on a street level, where people think they can get away with it. We need to challenge the rise in intolerance, and welcome those coming from abroad,” he said.