FOUR of Britain’s opposition parties today seek an alliance with the trade union movement to widen their campaign for the country to stay in the European single market and customs union.

The leaders of the SNP, Liberal Democrats, Plaid Cymru and the Greens have invited Frances O’Grady, General Secretary of the TUC, to attend the second meeting of their single market summit at Westminster next month.

It is thought that in the coming days a similar invitation will go out to Sadiq Khan, the Labour Mayor of London, as the soft Brexit campaign seeks to widen its appeal beyond Parliament.

The move comes as Sir Nick Clegg, the former Deputy Prime Minister, argued that MPs would have a “solemn democratic duty” to vote against the final Brexit deal, if it did not deliver all the benefits promised by the Brexit campaign during the 2016 EU referendum campaign.

In a letter to Ms O’Grady, the party leaders expressed their agreement over concerns that workers’ rights, jobs, incomes, and living standards were all under threat from “extreme Tory plans to drag the UK out of the single market”.

Backing the TUC campaign to protect the Working Time Directive, they warned that single market membership was crucial to prevent the Tories watering down or scrapping the hard-won employment rights that millions of people across the UK currently enjoyed such as paid holidays, lunch breaks, reasonable working hours and parental leave entitlements.

The opposition leaders believe trade union support is “key” to building a majority at Westminster and across the country for remaining in the single market.

Earlier this month, Jeremy Corbyn declined to be part of the soft Brexit alliance, branding it “a stunt”. He was empty-chaired at the group’s first meeting but Ian Blackford, the SNP leader in the Commons, has admitted that without the Labour leader on board it is unlikely the campaign will succeed.

He said of the parties’ attempt to get the TUC on board: “Frances O’Grady is a champion for workers and the trade union movement has a crucial role to play in building support for the single market in parliament, and across the country.

“As we hurtle towards the EU exit door, it is incumbent upon all of us who care about the rights we enjoy and the long-term prosperity of the country to do everything we can to prevent the devastation a Tory hard Brexit would cause.”

Sir Vince Cable MP for the Liberal Democrats, said: “Frances O’Grady is a widely respected figure among all parties and has been staunch in defence of the single market and customs union. As TUC leader, her words are poignant reminders to the Labour leadership, which continues to let down workers by not joining us in opposing the Conservatives.”

Caroline Lucas, co-leader of the Green Party, said: “Leaving the single market would be a disaster for workers. Not only are thousands of jobs dependent on it but it also enshrines workers’ rights across national borders and avoids a race to the bottom. The TUC are a key voice in the fight to stay in the single market and have a vital role to play as a part of this struggle.”

Meanwhile, Sir Nick told BBC Radio’s Westminster Hour that the Commons vote on the UK Government’s Brexit deal would be “the most momentous vote in Parliament, certainly since the vote on the Iraq war, possibly the most momentous vote in the post-war period”.

He urged MPs to reject the deal, saying it would not deliver the promises made to people by the Leave side in the EU referendum campaign.

“What I hope many MPs will do from across the political divide is simply compare what their constituents were promised – extra money for the NHS, smaller class sizes, VAT reduction, a cornucopia of instant new trade deals which would more than outweigh any lost trade that we have with the European Union – to what the deal actually includes.

“I safely predict that not a single one of those commitments made to millions of constituents across the country will be included in that deal,” added the former Liberal Democrat leader.

In a separate development, the Commons Treasury Committee in its report on the 2017 Budget has today called on the Government’s independent forecaster, the Office for Budget Responsibility, to publish a “special forecast” before the introduction of the Withdrawal Agreement and Implementation Bill to “ensure Parliament is fully informed about…[the]economic and fiscal impacts” of Brexit on economic growth.