Differences have emerged at the top of the campaign to keep Britain in the European Union, after Home Secretary Theresa May used her first major intervention in the referendum debate to argue that Britain should stay in the 28-nation bloc, but pull out of the European Convention on Human Rights (ECHR).
Mrs May's comments appeared to put her at odds with Prime Minister David Cameron, who has previously said he is seeking to reform the Convention, but rules nothing out if this proves impossible.
Asked whether Mrs May was speaking on behalf of the Government, the PM's official spokeswoman would say only that she was "setting out her views as Home Secretary".
Brexit campaigners said that Britain could not ditch the Convention - which is operated by the separate Council of Europe - without breaching EU treaty obligations. And Liberal Democrat leader Tim Farron accused the Home Secretary of using the referendum debate to further her claim to be next Conservative leader.
Meanwhile, 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 the German city of Hanover ahead of a summit with Mr 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".
A retreat from European unity and the reconstruction of the "barriers and walls" which divided the continent in the 20th century would risk halting progress towards democracy and tolerance elsewhere in the world, the president warned.
"A strong, united Europe is a necessity for the world because an integrated Europe remains vital to our international order," he said.
Mr Obama's intervention came as Brexit's biggest hitters sought to regain the referendum initiative by putting immigration at the top of the agenda.
Justice Secretary Michael Gove warned the UK faces a migration "free-for-all" constituting a "direct and serious threat" to public services unless it breaks away from Brussels.
Writing in The Times, the Justice Secretary warned that 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".
Former work and pensions secretary Iain Duncan Smith said immigration was "out of control", while London mayor Boris Johnson said that Mr Cameron had achieved "two thirds of diddly squat" in negotiations for a special deal for Britain on free movement and other key demands.
But Mrs May insisted that "nobody should think" Brexit is the "single bullet that is suddenly going to solve all our immigration problems".
While free movement rules "mean it is harder to control the volume of European immigration", they "do not mean we cannot control the border," she said, in a speech in London.
"Remaining inside the European Union does make us more secure, it does make us more prosperous and it does make us more influential beyond our shores," said Mrs May.
By contrast, the ECHR adds nothing to UK prosperity and "makes us less secure by preventing the deportation of dangerous foreign nationals", she said.
"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," said Mrs May.
Downing Street said any discussions on new EU members were "years away" and that the UK had a veto over future enlargement.
Mr Cameron's renegotiation deal ensured that different transitional rules would be applied to nationals of any future member states, said the PM's official spokeswoman.
Mrs May appeared to signal a break with the UK's long-standing support for Turkish membership, asking whether now was the right time "to contemplate a land border between the EU and countries like Iran, Iraq and Syria".
Former cabinet minister and EU commissioner Lord Mandelson said the Vote Leave campaign had "hoisted the white flag on arguments around the economy" and was now running a "Ukip-lite strategy centred on immigration".
Former Cabinet minister Owen Paterson became the first prominent supporter of Brexit to suggest that the European question may not be settled by a victory for Remain on June 23.
While insisting he expected the Leave camp to win the national vote, the former environment secretary said that even a narrow victory for Remain would mean 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.
"You won't put that genie back in the bottle," said Mr Paterson.