A Tory MP yesterday demanded that Prime Minister David Cameron sack LibDem ministers who vote against the Coalition Government, as the tit-for-tat row between the two parties deepened.

The LibDems have announced they will retaliate against the loss of their cherished Lords reform plan by voting against proposals to cut the number of MPs in boundary changes.

The junior Coalition partner insists the two policies were linked and it would be wrong for one to go ahead without the other.

But it has raised eyebrows by saying it does not expect LibDem MPs to be fired for opposing the plan.

Under a convention known as collective cabinet responsibility, ministers are expected to vote for official government policies. Those who refuse to do so are usually sacked by the Prime Minister, or they pre-emptively resign.

But it is understood that the LibDems do not believe that they can be sacked en masse, not least because such a move would, by definition, mean the end of the Coalition.

But the Conservatives have refused to rule out firing all ministers who vote against the plan, which is also a Tory policy.

A Downing Street source said earlier this week "we will cross that bridge when we come to it".

Yesterday, Stewart Jackson, the Tory MP for Peterborough, said it would be "untenable" for LibDem ministers to vote against Government policy and remain in office – and that it would be "weak" of the Prime Minister to allow it.

The boundary changes are predicted to give the Conservatives an extra 20 safe seats at the next general election, expected in 2015. They are considered a crucial part of the party's campaign plan, not least because it is still smarting at being unable to form a majority government after the last election.

The move would change the size and shape of almost every constituency across the UK, as well as cutting seven Scottish MPs. The proposals are part of plans to cut MPs and equalise the number of voters.