Soldiers who object to taking part in a military occupation of a foreign country will face life in prison under measures due to be rubber-stamped in the House of Commons on Monday.

The little-noticed Armed Forces Bill will have its third reading in the Commons on Monday and left-leaning MPs are alarmed that it will legitimise pre-emptive military strikes.

It will change the definition of desertion to include soldiers who go absent without leave and intend to refuse to take part in a "military occupation of a foreign country or territory".

Under the current Army Act, desertion is defined as "going absent intending not to come back, going absent to avoid any service overseas, or going absent to avoid service when before an enemy".

Campaigners have seized on the inclusion of "military occupation" as evidence that ministers are trying to scare soldiers from objecting to future preemptive strikes - a charge denied by the Ministry of Defence.

The move comes as tensions escalate with Iran and follows a reported rise in soldiers absconding from the British Army since the invasion of Iraq.

Ben Griffin, a former SAS soldier who quit the army without facing punishment after being "appalled" at what was happening in Iraq, accused the government of changing the law ahead of any possible action in Iran.

"The government are kicking themselves in the teeth," he said. "Currently the British Army is a volunteer force, but using this sort of stick to beat soldiers into doing what they are told is turning it into a conscript army."

Mr Griffin, 28, from London, said that this would hamper morale and ultimately the effectiveness of the military.

Anti-war campaigners claim the change means it would expressly legitimise occupation and force soldiers to contravene the Nuremberg Principles, limiting their right to becoming conscientious objectors.

The Ministry of Defence has denied these charges and said that in fact the legislation introduces a new, lesser sentence of two years' imprisonment.

John McDonnell, the left-wing Labour MP, has tabled amendments to slash the maximum sentence for any desertion from life imprisonment to two years. He is also urging other MPs to throw out the new definition of desertion.

Angus MacNeil, the SNP MP who also spoke at the meeting, urged as many MPs as possible to debate the bill on Monday.

"There are a number of alarming measures in this bill that have appeared with little debate that need further scrutiny," he said.

The Ministry of Defence denied that the scope and definition was new or tougher than the existing law.

"Under current legislation the maximum penalty for all offences of desertion is life imprisonment. In most cases we have reduced this in the Armed Forces Bill to two years," a spokeswoman said.

However, where an offender deserted to avoid active service, such as the Iraq war, the maximum penalty would be applied, she said.