David Cameron is continuing to seek consensus in the House of Commons on UK involvement in air strikes in Syria, but cannot put a timescale on a vote, Downing Street has said.
Number 10 denied press reports that the Prime Minister has abandoned hope of winning parliamentary approval to extend RAF operations against the Islamic State terror group - also known as Isis, Isil and Daesh - from Iraq into Syria.
But the prospect of a vote - once expected soon after the Commons returned from its summer break - now appears to be receding, with military action opposed not only by Labour leader Jeremy Corbyn but also by a group of 20-30 Conservatives who are resisting Mr Cameron's efforts to win them over.
The PM's plans were dealt a further blow by a report from an influential Commons committee, which cautioned that he should not ask MPs to back action in the Middle Eastern state until he can show there is a clear plan both to defeat the jihadists and to end the bloody civil war.
The strongly worded report by the Foreign Affairs Select Committee warned that RAF strikes would only have a "marginal effect", but could be a "distraction" and compromise efforts to find a diplomatic solution to the crisis. The Tory-led cross-party committee said it was "not yet persuaded" that Mr Cameron would be able to address their concerns.
Chancellor George Osborne acknowledged there was currently no majority in the Commons for extending air strikes, but insisted the Government had not shelved plans for an eventual vote.
"Our position is very clear - we take the fight to Isis wherever we can," Mr Osborne told the BBC.
"But we are not going to go to the House of Commons unless we would be clear that we would win that vote and there would be a consensus for that action, and at the moment it's not clear that there is a majority for it.
"We continue to make the argument and when the time comes, we will put the vote to the House of Commons."
Mr Cameron was bruised by a 2013 Commons defeat on military action against the forces of Syrian dictator Bashar Assad, and his official spokeswoman said the PM would only go back to MPs if he was sure of "support across the House".
The spokeswoman told a Westminster briefing: "The position has not changed. If you look at what the Prime Minister was saying last month on this, you can't put a timescale on the vote because that comes down to going back to the House when there is greater consensus across the House of Commons for that action.
"The PM is clear that there is a case for doing more and he will keep working, and the UK Government will keep working with allies to look at what we can do to protect ourselves and others from the threat of Isil. It is clearly for MPs that don't share those views to reflect on the threat from Isil and what we do to protect ourselves from it."
The FAC report acknowledged the "humanitarian and security catastrophe" in Syria meant there was a "powerful sense that something must be done", and that defeating IS was a "necessary goal for the UK".
But it added: "We believe that there should be no extension of British military action into Syria unless there is a coherent international strategy that has a realistic chance of defeating Isil and of ending the civil war in Syria.
"In the absence of such a strategy, taking action to meet the desire to do something is still incoherent."
The MPs said a bombing campaign would require a UN Security Council resolution and "reliable" allies on the ground to identify targets and hold ground. They demanded to be told how it would improve the chances of success against IS and contribute to a transition for Syria and whether it would be supported by regional powers including Turkey, Iran, Saudi Arabia and Iraq.
"We are not yet persuaded that it is possible for the Government to provide a satisfactory explanation of these points at present," they said.
"Until it can, we recommend that it does not bring to the House a motion seeking the extension of British military action to Syria."
Committee chairman Crispin Blunt said: "We are concerned that the Government is focusing on extending air strikes to Syria, responding to the powerful sense that something must be done to tackle Isil in Syria, without any expectation that its action will be militarily decisive, and without a coherent and long-term plan for defeating Isil and ending the civil war."
Committee member John Baron - who led the Tory rebellion over Syria in 2013 - said: "Fresh thinking, and not just more force, is needed on Syria. Otherwise, we risk making similar mistakes to those in Iraq, Afghanistan post-2006 and Libya. We need to focus our energies on defeating Daesh, and create a regional strategy which includes Russia and Iran. Air strikes alone will not achieve the desired effect."
However, Conservative MP Johnny Mercer - a veteran of the Afghan conflict - urged supporters of air strikes to "work harder" to make the case, warning it was "naive" to suggest there was no role for military action to complement the wider political and diplomatic effort to find a solution in Syria.
Mr Corbyn told ITV: "There has to be a political solution. I don't believe that more bombing is actually going to help in this.
"I think there is now a growing mood to do that between Russia, the United States and of course Iran, Saudi Arabia and Turkey. They all have to come together on this. Otherwise, we are going to end up with a proxy war going on, which will be even more desperate for the people of Syria."