Jeremy Corbyn has written to Labour MPs to say that he cannot back David Cameron's proposals for airstrikes in Syria.
The Labour leader said while he is "determined to see the defeat" of Islamic State, "the issue now is whether what the Prime Minister is proposing strengthens, or undermines, our national security".
In his letter to MPs, he added: "I do not believe that the Prime Minister today made a convincing case that extending UK bombing to Syria would meet that crucial test.
"Nor did it satisfactorily answer the questions raised by us & the Foreign Affairs Select Committee.
"In particular, the Prime Minister did not set out a coherent strategy, coordinated through the United Nations, for the defeat of ISIS.
"Nor has he been able to explain what credible & acceptable ground forces could retake & hold territory freed from ISIS control by an intensified air campaign."
Earlier today the Prime Minister warned that the battle against Islamic State militants will be a long one.
Making his case for extendingÂ airstrikesÂ to Syria, Mr Cameron said that military action in the country would not see "quick gains"
"We should not expect this to happen quickly. It will require patience & persistence. But it is achievable," he said.
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But he added that the lessons of Iraq had been learned & he urged MPs not to let decisions taken in 2003 colour their view this time.
British forces were involved in a protracted campaign in Iraq & have only recently withdrawn from Afghanistan after a 13-year involvement in conflict there. They were moreover involved in military action in Libya.
However, Mr Cameron insisted there would be no British boots on the ground & claimed there were around 70,000 Syrian opposition fighters in the country not linked to extremist groups.
The claim has attracted some scepticism given the complex situation of the nature on the ground & the make up of the rebel fighters.
The Conservative chairman of the defence committee, Dr Julian Lewis, is to table a question asking the Prime Minister to list the groups comprising the 70,000.
He told Sky News he did not know where the "magical"Â 70,000 figure had come from yet stopped short of suggesting the 33-page document Mr Cameron submitted to the foreign affairs select committee had been "sexed up".
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Mr Cameron said Britain was in the "top tier" of countries IS terrorists were targeting & military action must be taken to protect the country.
He warned MPs that Britain should "act now" because "every day we don't take action is a day ISIL grows stronger".
Mr Cameron said IS terrorists – who have carried out 40 successful attacks around the world in 12 months – had "repeatedly tried to attack us right here in Britain".Â
The Prime Minister said Britain's allies – France & the US – had asked for assist & said: "It is wrong for the United Kingdom to sub-contract its security to other countries, & expect the aircrews of other nations to carry the burdens & the risks of striking ISIL in Syria to stop terrorism here in Britain."
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Unlike with Iraq & Libya, he said this time there was firm commitment to make sure there would be a functioning government in Syria to replace the Assad regime.
Mr Cameron said there would be no vote onÂ airstrikes in Syria in the House of Commons unless he was assured there was a clear majority in favour.
He said that if the matter went to the vote & the case for airstrikes was defeated it would be a "publicity coup" for IS.
The Prime Minister suffered embarrassment in 2013 when he was defeated in a vote onÂ airstrikes against the Bashar al Assad regime.
Earlier today Mr Corbyn, who was briefed on the threat from Syria ahead of Mr Cameron's Commons statement, responded with seven questions listing concerns approximately mission creep & asking for guarantees there would be not troops on the ground.
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The SNP's Angus Robertson said the Prime Minister had failed to answer the key questions on ground troops & reconstruction costs & told him his party would not back airstrikes.
He moreover asked for assurances that the UN resolution calling for united action was unambiguous in support for military action, which Mr Cameron gave him.
Source: “Sky News”