Labour has called on the SNP to vote with it against plans to amend foxhunting laws south of the Border, inflicting the first major Commons defeat on David Cameron's Conservative government.

Tens of thousands have also signed an internet petition urging the SNP to oppose what campaigners say would be a return to traditional foxhunting by the back door.

Tory ministers say the move would merely bring the law into line with Scotland - and not affect what happens north of the border.

But critics claim that the small print of the reforms contains changes that could have knock-on effects for Scotland.

With a close result expected on Wednesday, the SNP's 56 MPs could swing the vote.

The party traditionally does not vote on "England only' issues.

Opponents to fox hunting are suspicious that theConservative's announced plans they claim will bring the law into line with in Scotland in part to convince the SNP to abstain.

One SNP source, however, said that the party would make up its own mind how to vote.

The party was not minded to "offer comfort to the Conservatives" by setting out how it would vote at an early stage, he added.

SNP MPs are due to meet to discuss the issue later today.

The SNP, and other Scottish MPs, are also currently involved in a separate Commons battle with the Conservatives over what laws are deemed 'England-only'.

SNP MPs were outraged last week when the Commons leader Chris Grayling suggested Scottish MPs should be barred from voting at Westminster on devolved issues.

They argue that policy decisions on issues such as health can have a huge impact, often financial, on Scotland.

Labour will oppose the fox hunting changes, while the Conservatives have been given a free vote.

A number of senior Conservatives, including Tracey Crouch, the sports minister, are expected to vote against the government.

Experts predict that the final result could be extremely close.

In a letter to the SNP's Westminster leader Angus Robertson, Labour frontbenchers Ian Murray and Maria Eagle said: "Many people, including in Scotland, will oppose any move by the Government to water down the existing law in England.

"We are sure you, and members of the SNP group, will have already received representations from constituents and others in this regard. It is important that all of us listen to people across Scotland who have been making their views clear since this vote was announced last week."

They also described the move as the "re-introduction of hunting with dogs by the back door".

And they added: " Our resolve against these immoral practices should not stop at the border."

An SNP spokesman said the party would examine the details of the Government's proposals before deciding what to do.

An SNP spokesman said: "As with all proposals from the UK Government, SNP MPs will decide our position once we have assessed the detail of what is being proposed."

The party declined to comment on Labour's call.

It came as the Scottish Government ordered MSPs to investigate the effectiveness of the hunting ban in Scotland, which is being flouted by hunts which allow foxes to be killed by dogs.

Environment Minister Aileen McLeod has informed the League Against Cruel Sports Scotland of the move following a presentation the organisation made at Holyrood earlier this summer.

Meanwhile, more than 100,000 people have signed an internet petition asking the SNP to vote against the changes.

Yesterday the petition, on the Care2 petitions website, had 105,300 signatures.

Traditional fox hunting with dogs is illegal across Britain.

In England and Wales just two dogs can be used to flush out a fox so that a farmer or landowner can shoot it as part of pest control measures.

In Scotland an unlimited number of dogs can be used.

Former environment secretary Owen Paterson clashed with animal rights activist and Queen guitarist Brian May over the issue yesterday.

Mr Paterson said the proposals were for a "minor technical amendment" which would help farmers.

But Mr May told him: "This is about... people rushing around the countryside torturing wild animals to death for fun, that's what this is about and it is an absolute scam to pretend that this is about farming in the first place."

He added: "This is an excuse, Mr Paterson, to go out there and be fox hunters again. It is a criminal act, you are talking about a criminal act of cruelty."

Wednesday's vote will be held just before MPs debate plans to restrict Scottish MPs voting rights.