Almost half of Conservative activists now want David Cameron to forge some sort of pact with Ukip before the 2015 general election, in a sign of mounting nervousness over the threat they face from Nigel Farage's anti-EU party.

A poll of more than 1,000 activists taken by the ConservativeHome website and obtained by the Observer found that 41% favour a pact, while 54% reject the idea. The number in favour has risen by 7% compared with last May, when the same questions were last asked.

The findings will alarm the Conservative leadership, which has strongly rejected any form of pact with Ukip, even though several Tory seats are seen as definitely at risk from Farage's party splitting the rightwing vote.

When asked what kind of pact they would favour, 17% of those questioned favoured a vote-swapping arrangement similar to that suggested by the journalist Toby Young, while 16% wanted an arrangement under which Conservatives and Ukip candidates would stand down for one another depending on which was more likely to win a particular seat. A deal that would give Ukip specified roles in government, such as cabinet positions, in return for its candidates standing down was backed by 7%. A plan for joint Ukip-Tory candidates was supported by 6% and a full merger of the two parties by 7%.

The Tory backbench MP Jacob Rees-Mogg angered the leadership last year by suggesting his party should allow Ukip to win parliamentary seats in order to avoid splitting the Eurosceptic vote.

Young has said he wants "a patchwork quilt of constituency-level agreements between Ukip and Tory activists whereby they agree to vote for whichever party's candidate is best-placed to win in 2015 in certain key marginals".

The former Conservative MP Paul Goodman, who now edits ConservativeHome, said the poll showed that more and more Conservatives were forming the view that Cameron would be unable to win an overall majority alone, such was Ukip's appeal and strength. "This shows that a big tranche of Conservative activists believe David Cameron can't win a majority on his own, or that he should govern with Ukip in any event, or both," he said.

Goodman said that in his view a pact with Ukip in the short term would be "both impracticable and undesirable – and a plurality of Tory members don't want it anyway". But after an election, some form of arrangement might be good for the party. "In the medium-term, that's to say post-2015, some sort of accommodation with the more pragmatic elements in Ukip will probably be necessary," he said. "The Conservatives should be trying to build the biggest electoral tent possible – and that means winning support from their right as well as their left, not to mention uncommitted and alienated voters."

Cameron's difficulties over Europe mounted last week when the French President François Hollande made clear that he would not agree to major changes to the EU treaties, as the prime minister wants, in time for an in/out referendum by the end of 2017.

Hollande's resistance to the idea of re-opening the EU rulebook underlines the scale of the difficulty Cameron faces in persuading other EU nations to commit time and energy to satisfying UK demands.

An Opinium/Observer poll shows Ukip unchanged on a fortnight ago on 17%, with the Tories down one percentage point on 29% and Labour unchanged on 36%. The Liberal Democrats are unchanged on 8%.

A Ukip spokesman said the party was against any formal arrangement, although if a local association believed a sitting MP from any party was sufficiently Eurosceptic, local officials could discuss whether a local agreement might work with the national party.

George Osborne has explicitly ruled out any form of deal with Ukip and has hinted that any MP who tries to advance such plans will be disciplined.