David Cameron has indicated he will abandon plans to extend military airstrikes against Islamic State (Isis) targets from Iraq to Syria if Jeremy Corbyn is elected leader of the Labour party.

In a sign of how the leftwinger could have an impact on Britain’s foreign policy, even as opposition leader, the prime minister said that he would only hold a parliamentary vote on the strikes if there is “genuine consensus”.

Speaking in Madrid, the prime minister said: “I would only proceed going further on this issue if there is genuine consensus in the United Kingdom about it before going back to parliament.” The frontrunner for the Labour leadership is opposed to the current airstrikes against Isis targets in Iraq and to extending them to Syria.

Corbyn also suggested in the last Labour leadership hustings, broadcast on Sky News on Thursday evening, that he could not currently envisage the circumstances in which he would agree to deploy British forces. The prime minister has a longstanding commitment not to extend the bombing campaign without the approval of MPs, even though he is technically free to bypass parliament on the deployment of military forces by using the royal prerogative.

Michael Fallon, the defence secretary, appeared to pave the way for a parliamentary vote in the early autumn when he said it was illogical for British airstrikes to be targeted at Isis forces in Iraq but not in Syria. MPs approved the airstrikes against the targets in Iraq after Britain was invited to join the campaign by the Iraqi government.

Tory sources suggested over the summer that a parliamentary vote could be held as early as this month to serve as a test for the new Labour leader, who will be elected on 12 September. This plan was drawn up when No 10 thought one of the mainstream candidates – Yvette Cooper, Andy Burnham or Liz Kendall – would be elected leader. Downing Street hoped that as leader, Cooper, Burnham or Kendall, who have raised questions about extending the strikes but have not rejected them out of hand, would be nervous about appearing to be weak on security.

But the emergence of Corbyn, who has rejected the strikes, appears to have led to a rethink in No 10. Advisers fear there could a repeat of the events of August 2013, when the prime minister was defeated in his attempts to win backing for airstrikes against the regime of Bashar al-Assad after Ed Miliband declined to support the government.

The prime minister would need the votes of a sizeable number of Labour MPs because around 20 Tory MPs would probably vote against an extension of the airstrikes.

Cameron said in Madrid: “We support the action that is being taken against Isil [Isis] in Syria. We are directly carrying that out in Iraq where the RAF have carried out a series of strikes with allies and degraded and set back Isil. I believe that process should continue and will continue.

“But we already support what is happening in Syria. We are providing intelligence, we are providing air support, we are providing refuelling support. We back what is being done by the coalition because it is vital we degrade, we set back and ultimately destroy Isis in both countries.

“But I would only proceed going further on this issue if there is genuine consensus in the United Kingdom about it before going back to parliament.”



