The UK would be irretrievably damaged and could face separation if the electorate votes to leave the European Union in the referendum planned by David Cameron after next year’s general election, a leading pro-Europe Tory has said.

In a sign of more aggressive tactics by Pro-Europe Tories before the referendum planned for 2017, Laura Sandys, chair of the European Movement, said a British exit would prompt another Scottish independence referendum and destabilise Northern Ireland.

“Within years [the UK] would be irretrievably damaged, politically and economically,” she writes in an article for theguardian.com.

The warning from Sandys – whose father, Duncan, founded the European Movement, a pro-Europe pressure group, in 1947 – echoes a blunt message from the Scotland secretary, Alistair Carmichael. In a Guardian interview for a recent series on the Scottish referendum, Carmichael predicted that a vote to leave the EU would mark the “ruination” of the UK.

Leading Labour figures in Scotland who were involved in the Better Together campaign to save the union have echoed Carmichael’s fears in private. One senior figure told the Guardian that it would be difficult to sustain Scotland’s position within the UK if the country as a whole voted to leave the EU.

Sandys writes: “Withdrawal would not support a stronger union because pro-European Scotland would be back at the ‘independence’ ballot box within months.”

She said a vote to leave would also have an impact on Northern Ireland, the only part of the UK to have a land border with another EU member state.

“It is Northern Ireland’s future stability that would be most compromised; all political understandings would be threatened, the economy would be destabilised, and cross-border trade totally compromised. A political impact assessment of the consequences of European withdrawal for Northern Ireland would be a terrifying document to read, for the people of Northern Ireland and the coherence of the UK,” Sandys writes.

Nicola Sturgeon, Scotland’s first minister, has called for voters in each of the four constituent parts of the UK to be given a veto over a British exit from the EU. This was rejected by Cameron, but many Labour figures in Scotland believe that Sturgeon has positioned the SNP with some skill on the grounds that nationalists would be handed a gift if Scotland voted to remain in the EU while the UK as a whole voted to leave.

Pro-Europea activists have been strengthening their operations, concerned that Eurosceptics and outright opponents of the EU have been allowed to dominate the debate as Nigel Farage rides a populist surge. Peter Mandelson and Kenneth Clarke have agreed to join forces with the Liberal Democrat cabinet minister Danny Alexander to serve as co-presidents of British Influence, which is likely to become the main pro-Europe campaign group in any referendum.

Sandys, the MP for Thanet South since 2010, who has decided not to contest the seat where Farage will be standing next May, accuses opponents of the EU of embodying “a strange combination of defeatism and lack of ambition”.

She describes advocates of withdrawal as “bolters”, and adds: “They pose as the great patriots, but would any prime minister or British monarch (at least since Elizabeth I refused to marry a continental king) develop a clear policy to reduce the UK’s influence in Europe? That is what exit means – a real and highly substantive reduction in the UK’s influence with our neighbours and in our power to shape Europe’s policy.

“I do not see how anyone could think that a UK sitting on the outside would be a more powerful country; that withdrawal from any international club would enhance our international influence; or that leaving a trading organisation could in anyone’s mind reflect an economic strategy that will serve British business and jobs better.”