The Government made a last-minute appeal to Labour MPs yesterday not to support a ban on fox hunting in England and Wales in a critical vote in the Commons tonight.

There are growing signs that Labour backbenchers will defy Tony Blair by backing an amendment that would toughen the Hunting Bill, even though this could delay its passage through Parliament.

The Government's proposals would ban hare coursing and stag hunting, but would allow fox hunting under strict conditions. But more than 140 MPs have backed a rebel amendment tabled by Tony Banks, a former sports minister, calling for a ban.

Margaret Beckett, the Secretary of State for Environment, Food and Rural Affairs, said yesterday: "The Bill as it stands is acknowledged by animal welfare organisations to be the strongest ever put forward. No Bill on a simple ban has ever been thought to be workable."

She added: "MPs on a completely free vote can choose between what is simple to explain and what is simple to enforce. If cruelty is the main concern, I plead with colleagues neither to wreck the Bill, nor delay its timing."

Alun Michael, the minister for Rural Affairs, said pro-hunting MPs could prolong the suffering of foxes by voting for a complete ban.

He said: "That could be a problem. It could involve a delay whereas the Bill as it is offered deals with the issue of cruelty whole-heartedly. It is a complete ban on cruelty."

A Commons vote tonight for a ban could spark a bitter dispute between the Government and Labour MPs opposed to fox hunting.

Ministers angered them last week by warning that an amended Bill would have to be referred back to an MPs' committee, delaying its passage into law.

Labour whips were also warned last week that if the measure was amended to ban fox hunting, the Government's lawyers did not believe the Parliament Act could be used to force the Bill through the House of Lords, despite earlier assurances that it would be an option.

Kate Hoey, another former sports minister, said yesterday that anti-hunting "zealots" could wreck the Bill. She told GMTV that the "middle way" option of licensed hunting was no more cruel that other methods of controlling foxes.

She said: "I have always opposed a ban on hunting. It's ridiculous that we're spending eight or nine hours on it tomorrow when there are so many other problems."

Lembit Opik, a Liberal Democrat member of the "Middle Way" group, which calls for tighter regulation of hunting, said: "The pro-ban lobby are going to try and play hardball with those of us who think that a ban isn't the right way to go.

"It remains to be seen whether the moderates, like myself, can actually talk the extremists like Tony Banks out of forcing an amendment through which bans hunting with dogs. I don't think it makes sense but that's the battleground."

Last night, women and -children from the Countryside Alliance, which opposes a ban on hunting, began a vigil outside the House of Commons. The League Against Cruel Sports said of the vigil last night: "Hunting with dogs is a ritualised form of animal abuse for sport, thinly disguised as pest control. Most women will be disgusted by this blatantly sexist vigil in support of animal abuse for fun."