Alex Salmond has accused ministers of "opportunistically" using the Tunisian terror attacks to bolster their case for airstrikes in Syria.
The former First MInisters comments came as Labour signalled it was ready to back further military action in the wake of last week's atrocity.
Labour along with Tory rebels prevented UK military action in Syria in a dramatic Commons showdown two years ago.
But the party suggested it could back action now, provided it was legal and had the support of other states in the region.
Earlier Defence Secretary Michael Fallon had urged MPs to "carefully consider" how the UK tackles Islamic State (IS).
He warned that the terror group did not respect territorial borders.
But former SNP leader Mr Salmond accused ministers of using the Tunisian attacks to enhance its argument for pursuing an airstrike campaign.
Mr Salmond said: "I think the Defence Secretary was entirely wrong to rather opportunistically use the atrocity in Tunisia as an opportunity to revisit, what he believed anyway, which was to extend an air war into Syria.
Ministers alleged plans to pursue a bombing campaign in Syria were "ill-thought out" and unjustifiable "in the general cloak of international opinion", he added.
In recent days David Cameron has said that IS must be destroyed in its Syrian heartland.
Mr Fallon made clear that ministers would not stage a new vote on the issue unless it was apparent that there was "some consensus" among MPs.
But he said the United States and others were already carrying out air strikes in Syria and questioned whether the UK should do more.
"We have air strikes being carried out by Canadian aircraft that are, of course, helping to keep our streets safe as well," he said.
Mr Cameron's official spokeswoman said there was a need for "more thought, more deliberation, more time" before any decision was taken, although she stressed that the Prime Minister still believed that ultimately IS had to be dealt with in Syria.
"The PM's views haven't changed. What has changed is the growing evidence that (IS) represents a threat to Britain and our national security," she said.
No 10 pointed out that the UK was already providing reconnaissance and air-to-air refuelling capacity for the Syrian operation.
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