Britain’s trade unions have told Theresa May she must give the public a vote on whatever Brexit deal she negotiates with the EU.

The Trades Union Congress said it was “serving notice” to the prime minister, as it accused her of “holding the country to ransom” by insisting her Chequers plan is the only alternative to a no-deal Brexit.

The proposals would be “devastating’ for working people, the TUC said.

The body, which brings together unions representing more than 5.5 million workers, demanded Ms May call a general election or a Final Say referendum to seek public approval for her Brexit deal.

And it threatened to throw its weight fully behind growing calls for another referendum unless the prime minister abandons plans for a hard Brexit.

The major announcement by Britain’s trade unions will add to pressure on Jeremy Corbyn to shift Labour’s own policy towards backing a vote on the Brexit deal.

The Labour leader appeared to leave open the possibility earlier in the week when he said of the prospect of another vote: “We don’t have a position on it yet.”

Speaking in Manchester at the start of the 150th congress, Frances O’Grady, the TUC general secretary, said Ms May must allow the public to decide whether the deal she negotiates is “good enough”.

She said: “I’m a trade unionist. When we do a deal, we go back to the members and have to get their approval.

“Whether it’s through a general election or a popular vote, Ms May should take her deal on the terms of Brexit and put it back to the people, so the people can decide whether that deal is good enough.”

Demanding the prime minister adopt a softer stance and keep Britain closely tied to the EU, Ms O’Grady said: “No deal would be devastating for working people, but so would the prime minister’s proposals.

“And if Theresa May takes a ‘my deal or no deal’ approach, she won’t be giving parliament a real choice.

“She is holding the country to ransom, and no one voted for that. So today I want to give the prime minister fair warning. I am serving notice: that if her deal doesn’t deliver for working people, if it doesn’t protect jobs, rights at work and peace in Northern Ireland, then the TUC will throw our weight behind the call for a vote on the terms of Brexit.”

People's Vote march – demanding vote on final Brexit deal Show all 30 1 /30 People's Vote march – demanding vote on final Brexit deal People's Vote march – demanding vote on final Brexit deal Mandatory Credit: Photo by Guy Bell/REX (9725259aq) Marching down Whitehall - People's March for a People's Vote on the final Brexit deal. Timed to coincide with the second anniversary of the 2016 referendum it is organised by anti Brexit, pro EU campaigners. People's March for a People's Vote, London, UK - 23 Jun 2018 Guy Bell/REX Rex People's Vote march – demanding vote on final Brexit deal Demonstrators carry banners and flags as they participate in the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, the second anniversary of the 2016 referendum. Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal A young protestor shouts as she takes part in the People's Vote demonstration against Brexit Getty Images People's Vote march – demanding vote on final Brexit deal A woman holds a placard as she joins EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participating in the 'People's Vote' march in central London, Britain June 23, 2018. REUTERS/Henry Nicholls HENRY NICHOLLS Reuters People's Vote march – demanding vote on final Brexit deal epa06833888 Pro-EU demonstrators take part in a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal A protester's pro-EU t-shirt EPA People's Vote march – demanding vote on final Brexit deal epa06833892 A pro-EU demonstrator waves a European Flag during a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Demonstrators carry banners and flags as they participate in the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, the second anniversary of the 2016 referendum. Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal epa06833960 Tens of thousands of people march through London during a People's March anti-Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Leader of the Liberal Democrats, Vince Cable speaks during the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, the second anniversary of the 2016 referendum. Tens of thousands of people demonstrated in London on Saturday calling for a second vote on Britain's departure from the European Union. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty People's Vote march – demanding vote on final Brexit deal epa06833964 A young pro-EU demonstrator takes part in a People's March anti-Brexit rally in Parliament Square in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Gina Miller and Caroline Lucas EPA People's Vote march – demanding vote on final Brexit deal epa06834044 Tens of thousands of people march through London during a People's March anti-Brexit demonstration, in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal epa06833264 Thousands of people march through London during a People's March anti Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Tens of thousands of people march through London EPA People's Vote march – demanding vote on final Brexit deal epa06833265 Thousands of people march through London during a People's March anti Brexit demonstration in London, Britain, 23 June 2018. Protesters are calling for a referendum on the final deal. The protest is taking place on the second anniversary of the Brexit referendum. EPA/ANDY RAIN ANDY RAIN EPA People's Vote march – demanding vote on final Brexit deal Demonstrators at the People's Vote March Getty People's Vote march – demanding vote on final Brexit deal 'Two months too young to decide on my future' REUTERS People's Vote march – demanding vote on final Brexit deal Crowds taking part in the People's Vote march for a second EU referendum at Trafalgar Square in central London. PRESS ASSOCIATION Photo. Picture date: Saturday June 23, 2018. See PA story POLITICS Brexit. Photo credit should read: John Stillwell/PA Wire John Stillwell PA People's Vote march – demanding vote on final Brexit deal A young girl joins in the march PA People's Vote march – demanding vote on final Brexit deal An EU flag is draped across the statue of Winston Chruchill in Parliament Square REUTERS People's Vote march – demanding vote on final Brexit deal Demonstrators gather prior to the start of the People's March demanding a People's Vote on the final Brexit deal, in central London on June 23, 2018, on the second anniversary of the 2016 referendum. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images NIKLAS HALLE'N AFP/Getty Images People's Vote march – demanding vote on final Brexit deal Vince Cable MP, Pro-EU campaigner Gina Miller, Tony Robinson and Caroline Lucas MP join with crowds PA People's Vote march – demanding vote on final Brexit deal Crowds gather on Pall Mall in central London, during the People's Vote march for a second EU referendum. PRESS ASSOCIATION Photo. Picture date: Saturday June 23, 2018. See PA story POLITICS Brexit. Photo credit should read: John Stillwell/PA Wire John Stillwell PA People's Vote march – demanding vote on final Brexit deal Crowds gather on Pall Mall PA People's Vote march – demanding vote on final Brexit deal A man resembling Britain's Foreign Secretary Boris Johnson, joins EU supporters Reuters People's Vote march – demanding vote on final Brexit deal EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participate in the 'People's Vote' march in central London, Britain June 23, 2018. REUTERS/Henry Nicholls HENRY NICHOLLS Reuters People's Vote march – demanding vote on final Brexit deal People gather in Trafalgar Square REUTERS People's Vote march – demanding vote on final Brexit deal Pro-EU campaigner Gina Miller and Tony Robinson PA People's Vote march – demanding vote on final Brexit deal EU supporters, calling on the government to give Britons a vote on the final Brexit deal, participate in the 'People's Vote' march REUTERS

Several unions, including the GMB and TSSA, have already called for a second vote on Brexit, while more than three-quarters of a million people have now backed The Independent’s campaign for a Final Say referendum.

And a new poll today revealed members of Britain’s three biggest unions – Unite, Unison and the GMB – back a new referendum by a margin of more than two to one.

Arguing that the Brexit process is now at “crisis point”, Ms O’Grady said Ms May had “let Conservative Party politics take priority over the interests of working people” and accused her of being “too weak to stand up to the hardliners in her party”.

She called on the government to try to extend the Article 50 period to allow more time to avert a no-deal Brexit.

Ms O’Grady, who previously said she has met German chancellor Angela Merkel more than she has Ms May, also said she was “disappointed” at the prime minister’s lack of engagement with Britain’s unions on Brexit.

She said: “It’s more than disappointing that union leaders, representatives of nearly 6 million workers, have not yet been heard on Brexit. Where is the voice of workers?”

GMB General Secretary Tim Roache releases Brexit statement

She was speaking after the TUC’s general council agreed a lengthy statement outlining its position on Brexit.

The council said: “In the face of so much instability and mistrust in the ability of politicians to deliver, it comes as no surprise that polls show a growing number of people want a say on the Brexit deal.

“Given the real risk of a collapse in the talks, or a deal that does not deliver on the TUC’s priorities, whilst respecting the result of the 2016 referendum to leave the European Union we do not rule out the possibility of campaigning for people to have a Final Say on the Brexit deal through a popular vote.”

On Monday, union members at the conference will vote on a motion calling for a Final Say vote regardless of the deal Ms May brings back from Brussels.