London - Britain's approaching exit from the European Union could boost support for an independent Scotland and a united Ireland, according to surveys by pro-European campaign group Best for Britain.

Forty-seven per cent of Scots said they would support independence if a referendum were held after Brexit, while 43 per cent said they would vote to remain in the United Kingdom, according to the Deltapoll survey, which questioned 1,022 Scots.

However if Britain stayed in the EU, those figures would be reversed.

Scotland rejected independence by 55 to 45 per cent in a referendum in 2014. Scottish voters were also strongly in favour of remaining in the European Union in the 2016 referendum.

Support for a united Ireland

The survey also suggested that support for a united Ireland will jump when Britain leaves the EU, especially if a hard border is introduced between Northern Ireland and the Republic of Ireland.

Of the 1,199 Northern Irish polled, 52 per cent said they would vote for a united Ireland and 39 per cent to stay in the UK if a referendum were held after Brexit.

If a hard border were introduced 56 per cent would vote for a united Ireland, the poll suggested. If Britain remained in the EU however, support for a united Ireland would drop to 35 per cent.

Northern Ireland also voted overwhelmingly to remain in the EU in 2016.

"Clear and present danger"

Best for Britain chief executive Eloise Todd said the results were "compelling evidence as to why we need to stop and think again" on Brexit.

Opposition Labour parliamentarian Catherine Stihler said they showed "a clear and present danger to the future of the United Kingdom."

The survey came a day after Prime Minister Theresa May insisted that Britain would not hold a second referendum on the Brexit withdrawal agreement, saying it would be a "gross betrayal of our democracy."

She is meanwhile struggling to unite her own Conservative party around her Brexit proposals, agreed with her cabinet ministers in July and which are known as the Chequers agreement.

Battle with the white flag

Boris Johnson, who resigned as foreign secretary immediately after the agreement was reached, on Monday renewed his attack on her in a column in the Daily Telegraph newspaper, with British media reporting he could be preparing the ground for a bid to oust her.

Britain had "gone into battle with the white flag fluttering over our leading tank," Johnson argued, pointing to May's proposal to align British standards on goods to those of the EU.

He compared Brexit negotiations to a boxing match, writing, "The inevitable outcome is a victory for the EU, with the UK lying flat on the canvas and 12 stars circling symbolically over our semi-conscious head."

Other Conservatives who argue that the Chequers plan does not provide a clean enough break with the bloc are also expected to offer alternatives to May's proposals in the coming weeks.