A vote to keep the UK in the EU could be more important for ordinary British workers than the election of a reforming Labour government in 1945, Alan Johnson will say in a rallying cry to trade unions today.

The ex-Home Secretary and former trade union leader will point out how unions representing nearly four million workers have already pledged their support for continued EU membership; yesterday the Communication Workers Union was the latest to come out in favour of the Remain campaign.

In a speech to the Usdaw conference in Blackpool, Mr Johnson, who leads Labour’s In campaign, will say: “The vote in the referendum on the EU on June 23 is every bit as important as that election in July 1945; perhaps more so." This refers to the reforming post-war government of Clement Attlee, which created the NHS and expanded welfare.

He will say: "It is vital that our unions campaign for Britain to remain in Europe and campaign for a Europe that protects working people and keeps the swivel-eyed alliance of the right of the Tory party and Ukip off our rights at work."

Ten unions, with just short of four million members between them, have announced they will support the Remain cause.

Mr Johnson will add that the likes of Brexiteers Michael Gove and Boris Johnson know the EU protects worker’s rights and that was “one of the principal reasons why they want to leave the EU”.

As the Vote Leave campaign sought to switch attention to immigration following US president Barack Obama’s pro-EU intervention, Theresa May made her first intervention into the in-out referendum campaign.

In a speech in London, the Home Secretary said Britain should stay in the 28-nation bloc but leave the European Convention on Human Rights(ECHR).

She stressed how Britain had to "stand tall and lead in Europe" and that membership made the country "more secure from crime and terrorism".

But she questioned the EU’s expansion to the, noting how Albania, Serbia and Turkey had "poor populations and serious problems with organised crime, corruption, and sometimes even terrorism".

She explained: "We have to ask ourselves: is it really right that the EU should just continue to expand, conferring upon all new member states all the rights of membership?"

The Home Secretary stressed co-operation with other EU states on fighting crime helped improved Britain's safety but then claimed the ECHR "makes us less secure by preventing the deportation of dangerous foreign nationals".

She added: "So regardless of the EU referendum if we want to reform human rights laws in this country, it isn't the EU we should leave but the ECHR and the jurisdiction of its court."

But Tim Farron, the Liberal Democrat leader, hit back, saying the ECHR was fundamental to protecting Britons’ rights. "Theresa May's obsession with eroding human rights has reached a new low as the referendum debate becomes more about the Tory party leadership contest than the issues at stake,” he said.

Mrs May’s former cabinet colleague, Iain Duncan Smith, the anti-EU former Work and Pensions Secretary, also picked up on comments on EU expansion, noting how David Cameron was in favour of extending EU borders to Syria by allowing Turkey to join the bloc.

“This is a remarkable intervention; she has really thrown the gauntlet down and undermines the whole of the 'vote stay in' campaign," said the Scot.

Fellow Outer Owen Paterson, the former Environment Secretary, in his own speech in London, warned that the campaign to leave the EU would continue even if after a Remain victory, saying: “You can't put that genie back in the bottle."

His comments followed those of leading anti-EU campaigner Michael Gove, who claimed staying in the EU would result in a migration “free for all”.

The Justice Secretary warned the possible future accession of countries like Turkey and Albania would mean public services like the NHS facing "an unquantifiable strain as millions more become EU citizens and have the right to move to the UK".

But the pro-EU Lord Mandelson, the former Labour Business Secretary, said the Leave campaign had "hoisted the white flag on arguments around the economy" and was now running a "Ukip-lite strategy centred on immigration".