David Cameron's government was forced into its second humiliating climbdown in a week after SNP MPs scuppered moves to water down the fox hunting ban in England and Wales.
Ministers pulled plans for a Commons vote on the issue just hours after the SNP's 56 MPs confirmed they would oppose the changes.
The Prime Minister denied he had been "outfoxed" by SNP leader Nicola Sturgeon as he accused the SNP of "opportunism".
Ms Sturgeon hit back saying that the Tory leader's real problem was his tiny Commons majority of just 12 MPs.
In return, Downing Street said the Prime Minister remained "committed" to his manifesto pledge to allow MPs a vote on a full repeal of the hunting ban, suggesting that Tories hope the SNP's intervention will galvanise their own backbenchers.
The Conservatives ducked the vote rather than face potentially their first major defeat since winning May's General Election.
Less than a week ago ministers also postponed a vote on 'English votes for English laws' (evel) amid similar fears.
With up to 40 Tories expected to rebel on the fox hunting, the SNP's vote could have swung the balance.
The Tories had hoped that the SNP would abstain on the reforms, which ministers said would merely bring the law into line with that in Scotland.
Last night Labour party sources said they expected the Tories would have lost the vote even if the SNP had decided not to take part.
But on Monday night the the SNP announced it would vote against the reforms and asset the "Scottish interest".
Mr Cameron said: "The position of the SNP has up to now always been clear which is that they do not vote on matters that are purely of interest to England or England and Wales. I find their position entirely opportunistic and very hard to explain in any other way."
No 10 said that the Prime Minister was "disappointed that SNP involving themselves in an issue that does not affect Scottish people".
Ms Sturgeon said: "The reason David Cameron finds himself with SNP votes so pivotal here is that he can't carry his own parliamentary group on this.
"That's perhaps where his fundamental problem lies.
"This highlights how slender and how fragile the Conservative majority is."
Ms Sturgeon suggested earlier this year that her party would abstain on changes to the hunting ban south of the Border.
Yesterday she gave a number of reasons for changing her mind.
These included public opinion in England, the knock-on effects an amendment could have on Scotland and the opportunity to give the Tory Government a bloody nose, she said.
She also blamed David Cameron for trying to push through evel in a way she claimed was "unreasonable".
"Since the election, David Cameron's government has shown very little respect to the mandate that Scottish MPs have," she said.
But former Defence Secretary and Tory MP Liam Fox reacted angrily to the SNP's announcement, accusing the party of "sticking two fingers up" at the Government.
"People in England are perfectly capable of electing MPs to look after their interests. So we'll invite Nicola Sturgeon to keep her nose out of that one...,"! she said.
He also accused the party of "opportunism and hypocrisy".
In February Ms Sturgeon said that her party had a "longstanding position of not voting on matters that purely affect England - such as foxhunting south of the border, for example".
The Tory proposals would have allowed dogs to be used to to flush out foxes to be shot for pest control purposes south of the Border.
Traditional fox hunting is illegal across the UK.
In England and Wales at the moment only two dogs can be used to find foxes so that a farmer or landowner can shoot them.
In Scotland, however, an unlimited number of animals can be used.
Hunting supporters had backed the move, insisting that traditional hunting would still remain illegal.
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