The Tory sports minister, Tracey Crouch, has called on MPs of all parties to reject her own government’s plan to relax the 10-year-old hunting ban, saying they should not be “fooled by the spin of the pro-hunt lobby”.

Ahead of what may be a knife-edge Commons vote on Wednesday, Crouch is leading a group of prominent Conservatives who will side with Labour to maintain the law in its current form. David Cameron has promised a free vote on a government amendment that would make it legal to kill foxes for pest control and for them to be flushed out by a full pack of hounds. Under the current law, passed after years of parliamentary battles, hunts are allowed to use two dogs to flush out foxes that can then be shot for pest control purposes.

The changes follow a Tory manifesto commitment to offer a free vote on repeal of the law. But, fearing they would lose a vote for complete repeal, ministers have tabled changes to water down the bill, bringing the rules into line with those in Scotland.

Critics of the plans, which would also permit an unlimited number of dogs to flush out diseased or injured foxes and allow them to be used for “research and observation” purposes, say hunters would exploit the loose wording to claim they were assessing fox populations or disease levels when in fact they were hunting as they wished.

Crouch, who is a patron of the organisation Blue Fox – Conservatives Against Fox Hunting told the Observer that if MPs were lulled into backing the amendment they would be flying in the face of public opinion. “The proposed amendment relaxes current legislation making the current act unenforceable. The vote could be close, so I hope colleagues on all sides … will not be fooled by the spin of the pro-hunt lobby and inadvertently vote for a measure that makes a mockery of public opinion on foxhunting.” Between 20 and 30 Tory MPs could oppose the amendment.

When the ban became law in 2004, 61% of people supported parliament’s decision while 30% opposed it. Recent research by YouGov research found that 51% of people still support the ban while 33% oppose it.Sarah Wollaston, Tory MP for Totnes, said she too would vote against the amendment. “People have moved on from this. What this will do is make people think the Tories are all pro-hunting when that it is not the case.I also resent the way this is being presented as a town against country thing. In rural parts of my constituency many people do not agree with fox hunting.”

Justice minister Dominic Raab said recently: “I can’t see the case for repealing the Hunting Act. There are parts, like hunting deer with dogs, that I just feel are wrong in principle. With respect to foxhunting, I see that farmers need to protect their livestock from attack, but hunting foxes with dogs is not the most humane way to do that.”

The outcome could be determined by the 56 SNP MPs, who were refusing to say on Saturdayhow they would vote. It is expected that they will abstain, taking the view that it is a matter for England and Wales.

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Robbie Marsland, director of the League Against Cruel Sports (Scotland), said it was wrong to claim England was being brought into line with Scotland. What was being proposed in England was “worse” as it would allow full packs of hounds to be used for “research and observation”. For the UK government to maintain it was merely harmonising rules was also disingenuous because it implied the system in Scotland was working, when in fact the SNP was looking again at the shortcomings.

The Department for Rural Affairs said the proposed amendments to the hunting ban would enable farmers and gamekeepers to make a judgment, “based on the terrain and other circumstances, to use more than two dogs to flush out and stalk wild animals for effective and humane shooting as part of the existing exemption in the Act that allows for pest control”.