Environment secretary Hilary Benn has written to David Cameron calling on him to change Conservative plans to repeal the fox-hunting ban.
The open letter, which challenges the Tories' intention to give MPs a free vote on the ban, comes on the fifth anniversary of the Hunting Act passing into law.
Thousands of people added their signatures to the letter to Mr Cameron after a 24-hour campaign on the backtheban website.
"David Cameron's plan to repeal the hunting ban shows that beneath the gloss the Tories haven't changed," Mr Benn said.
"Five years ago, Labour banned fox-hunting, stag hunting and hare coursing because there is no place for animal cruelty in a modern, civilised society. Making animals tear each other apart is cruelty, not sport."
Mr Benn pointed out that Mr Cameron had appointed a former pro-hunting lobbyist as his 'animal welfare spokesperson'. He claimed this showed "how out of touch the Tories are in attempting to repeal such a popular, humane and effective law".
Mr Benn's claims that the ban has been successful seem vulnerable, given that according to the BBC there have only been three successful prosecutions under the Hunting Act since it became law.
But the International Fund for Animal Welfare today claimed over 100 people had been found guilty of offences under the Hunting Act.
An Ipsos Mori poll carried out in September 2009 revealed three-quarters of people support the ban on fox-hunting.
"The Act is clearly working and remains essential in deterring others from this now illegal behaviour," the RSPCA's director of animal welfare promotion John Rolls commented.
"It has modernised a traditional countryside pursuit by removing the element of animal cruelty previously associated with it."