THERESA May has called for Britain to remain in the European Union but leave the European Convention on Human Rights.

The Home Secretary’s first major intervention in the referendum debate came as the Leave campaign continued to denounce US President Barack Obama’s pro-EU support and sought to shift the debate from the economy onto fears of immigration.

While leading anti-EU campaigner Michael Gove, the Justice Secretary, claimed that staying in the EU would result in a migration “free for all,” his Tory colleague Owen Paterson, the former Environment Secretary and fellow Outer, warned that the campaign to leave the EU would continue even if there were a victory for the Remain camp, saying: “You can't put that genie back in the bottle."

In a keynote speech, Mrs May stressed how Britain had to "stand tall and lead in Europe" rather than leave the EU and argued membership made the country "more secure from crime and terrorism".

But she questioned the EU's expansion to the east, 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 also warned that no one should think Brexit was the "single bullet that is suddenly going to solve all our immigration problems".

While free movement rules meant it was harder to control the volume of European immigration, she insisted they did “not mean we cannot control the border".

Co-operation with other EU states on criminal records, biometric data and the European Arrest Warrant helped improved Britain's safety, she said, but then claimed the ECHR - which was an agreement by the Council of Europe and not the EU - "makes us less secure by preventing the deportation of dangerous foreign nationals".

Mrs May added: "So regardless of the EU referendum, my view is this: 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."

Her comments sparked a backlash

Tim Farron, the Liberal Democrat leader, claimed the ECHR was fundamental to protecting Britons’ rights, saying: "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.”

Ukip’s Diane James insisted the Secretary of State was "drowning in a sea of fact,” saying that as long as the UK remained in the EU there was “no escape” from Brussels’s control of human rights legislation.

Rachel Logan, of Amnesty said Mrs May’s proposal would strike at the very architecture of international protections and “betray the British people, who built the convention at the end of the Second World War”.

The Home Secretary's former cabinet colleague, leading Outer Iain Duncan Smith, also picked up on comments on EU expansion, saying they had "thrown the gauntlet down" to the Government.

Noting how the Prime Minister was in favour of extending EU borders to Syria by allowing Turkey to join the bloc, the Scot said: “She actually basically questions the very campaign she says that she supports. This is a remarkable intervention; she has really thrown the gauntlet down and undermines the whole of the 'vote stay in' campaign."

Across London, Mr Paterson used a speech to warn that the UK would be relegated to the status of a "colony of Europe" if it remained within the EU as the other countries in the bloc moved to a closer political union.

He said a narrow victory for Remain would have meant millions of voters supporting Leave, showing that anti-EU sentiment was a "very respectable" mainstream opinion rather than a view held by "nutters" on the fringes.

"Up to now,” he explained, “leaving the EU has been seen to be the preserve of nutcases, cranks and odd members of the right-wing of the Tory Party. If it is that close - I personally think we are going to win - millions of people from right across the country from every class and occupation will have stated that they want to leave the European Union. You won't put that genie back in the bottle."

In his Daily Telegraph column, fellow Brexiteer Boris Johnson warned the Remain side not to "crow too soon" that the Leave side had been "bombed into submission".

"The Prime Minister asked the EU for reform and got two-thirds of diddly squat. That deal shows how contemptuously we will be treated if we remain," declared the London mayor.

Mr Duncan Smith, the former Work and Pensions Secretary, insisted immigration was "out of control" and "poorer people" had seen their livelihoods damaged by EU citizens coming to the UK.

In a separate development, another leading union said it was to campaign for Britain to remain in the EU so that a "workers' voice" could be heard in the debate.

At its annual conference, the Communication Workers Union decided to recommend a Yes vote to its 200,000 members on the basis of reforming the EU to put workers at its heart.

Ex-Home Secretary Alan Johnson, who chairs Labour In For Britain, welcomed the move, saying: "It is great to have my former union backing the campaign to keep Britain in the European Union.

"As a former postman and CWU general secretary, I know the vital difference that union members can make to this campaign.

"The decision by CWU members highlights the unity of the Labour movement with all the biggest affiliated unions now campaigning for Britain to remain in the European Union,” he added.

Meantime, US president Barack Obama followed up his warnings on the economic risks of Brexit with an appeal for a "united" Europe to help drive global security, democracy and prosperity.

Speaking in Hanover ahead of a summit with David Cameron and other EU leaders, the American leader acknowledged that European unity involved "frustrating compromise" but hailed the multinational union as "one of the greatest political and economic achievements of modern times".

Mr Obama said: "I've come here today to the heart of Europe to say that the United States and the entire world needs a strong and prosperous and democratic and united Europe.

"A strong, united Europe is a necessity for the world because an integrated Europe remains vital to our international order," he added.