The ban on hunting foxes and deer with packs of hounds is in danger of being undermined, according to opponents of blood sports, after the government signalled its willingness to amend the 2004 Hunting Act.

In a little noticed parliamentary exchange last month, the rural affairs minister, George Eustice, confirmed that his department was considering relaxing restrictions on the number of dogs permitted to flush foxes towards guns.

The International Fund for Animal Welfare (IFAW), the RSPCA and other organisations fear any increase on the current limit of two will make enforcement of the ban on hunting mammals with hounds impossible.

Since the hunting ban came into force in 2005, there have been more than 400 prosecutions involving hunt officials and poachers using dogs. Despite initial reservations, animal welfare groups who monitor hunt activities believe the legislation is effective in enforcing the ban.

The Hunting Act permits a maximum of two dogs to flush a fox or other quarry towards someone who will shoot it. Animal welfare groups fear that if a whole pack of hounds is permitted to take part in such an exercise, while one person stands nearby holding a shotgun, it will be difficult to prevent a fox being torn apart and to prove subsequently it was intentional. On 13 February, Bill Wiggin, the Conservative MP for North Herefordshire, asked whether "it will be very popular with farmers when we amend legislation to allow more than two hounds to flush foxes to guns".

Eustice replied: "The government have had representations from ... Welsh farmers about the problems of predation, and there has been a proposal that the legislation be amended to increase the number of dogs that can be used for flushing out. We are looking carefully at the issue, and we will let [MPs] know when we reach any conclusions."

Lifting the restriction on the number of dogs would not require new legislation but could be altered through a statutory instrument that would, according to government sources, require votes in both houses of parliament.

Concentrating on such a narrow aspect of the legislation might be a way of finessing the prime minister's promise that MPs will be given a free vote on hunting before the end of this parliament. The Tories' failure to obtain a majority at the last election and the arrival of a small number of Conservative MPs opposed to hunting has resulted in the vote being repeatedly delayed. Opponents of hunting claim that amending the clause now would play well in the flood-devastated shires and bolster support for the party ahead of the European elections in May.

Robbie Marsland, the UK director of IFAW, which has been monitoring attempts to repeal the hunting ban, said: "Any attempts by the Conservatives to bring back hunting with dogs via an obscure amendment are merely to please a minority of his pro-hunt supporters and ignore the views of the vast majority of the UK public who rightly demanded this cruel activity be consigned to the history books.

"Ministers appear to be using underhand methods to make the Hunting Act unenforceable rather than calling for an honest vote on repeal that he surely knows he would lose. The majority of MPs across parties support keeping a ban on the chasing and killing of foxes, hare and deer with dogs for fun.

"If the cruelty of chasing and ripping apart sentient British mammals with packs of dogs for enjoyment is what ministers really want to reintroduce, they should be up-front about it and call for a vote on repeal – we are confident the UK public and the majority of MPs will rightly reject this abhorrent idea."

David Bowles, the RSPCA's head of public affairs, said: "This is the first indication that the government are finessing their position which was going to be to allow MPs a free vote. They want the situation as in Scotland where there's no limit on the number of dogs.

"This would be a wholesale amendment of the Hunting Act. It would drive a coach and horses through the ban. It's a more subtle approach than saying they are going for repealing the legislation."

A spokesperson for the Department of Environment, Food and Rural Affairs (Defra), said: "A number of Welsh farmers have brought this issue to our attention and we are looking at it."