Britain will not hold a second “In-Out” vote on its membership of the European Union (EU) if the public opt to leave the bloc at a referendum due by the end of 2017, a senior aide to Prime Minister David Cameron said on Sunday.

Some in the ‘out’ camp, including Vote Leave campaign director Dominic Cummings, have suggested that if Britain votes to quit the EU it could be used as a bargaining tool to get more concessions from Brussels ahead of another referendum, or a vote on the terms of the exit.

British media have reported that London mayor Boris Johnson is a supporter of the double referendum idea.

“The Prime Minister is clear that is simply not going to happen. From the outset, he has been clear this will be a straightforward in/out choice and that’s exactly what it will be. Leave means leave,” the aide told Reuters in an email briefing.

“It is not credible to suggest that the majority of the British public could vote to leave and then the UK government would ignore the voters and negotiate to remain.”

Cameron has promised to renegotiate Britain’s relationship with the bloc ahead of the vote, but many officials across the EU have expressed irritation that London has yet to spell out specific proposals.

The prime minister’s aide said the EU’s other 27 members would not countenance a second renegotiation with a country that has already decided to leave.

“Trying to muddy the waters as they are and suggest that we can have a second chance if we vote leave suggests that the Leave campaign lacks confidence in its own case and is worried about the risks. Otherwise it would unambiguously advocate a clean exit,” the aide said.

EU membership has long been a controversial issue in Britain, with Eurosceptics arguing that Britain would prosper more outside the bloc.

Public opinion is divided, however. Earlier this week a poll from Ipsos MORI showed British support for staying in the EU has tumbled over the past four months as an influx of migrants into Europe has pushed many voters towards opting for an exit.

The poll found 52 per cent of Britons would vote to stay in the EU, down from a record 61 per cent in June. Support for a British exit rose to 39 per cent, the highest level since 2012, up from 27 per cent in June. That more than halves the “in” lead to 13 percentage points from 34 points in June.