Commons leader Robin Cook has refused to deny suggestions that he will resign from the cabinet if MPs vote for an entirely appointed House of Lords.

Cook, who also has serious doubts about a war with Iraq, pointedly refused to dismiss suggestions that he may quit if the Commons rejects proposals for an elected element to the second chamber.

Speaking over the weekend, Cook warned that splits on Lords reform could result in there being no change to the composition of the upper house.

Cook, who backs a largely-elected second chamber, said that MPs must find common ground when they vote on the issue on Tuesday.

"I would advise colleagues to show some flexibility so that we can find common ground. What I don't want to see happen is that we end up with no reform because those that want reform can't agree with what sort of reform they seek," he told GMTV.

His intervention comes as cabinet in-fighting threatened to spill out into the open following Tony Blair's announcement that he backs an entirely-appointed second chamber.

Cook, who is said to be isolated in the cabinet, said he will continue to support a "largely elected second chamber".

"For the second chamber to carry credibility and authority, to make sure that it is legitimate, I believe that a large element of that second chamber has tobe elected - and I would invite colleagues to join me," he said.

The Commons leader added that voters would not support an appointed upper house.

"I think it's very important that we address the issue of the public trust in the parliamentary process...I personally believe that the best way of making sure that the public believes that the second chamber belongs to them is to enable the public to elect the larger part of that second chamber," he said.

"If they don't have any say in how the members of that second chamber are chosen, they are not going to feel that it's accountable to them."

He also refused to rule out resigning if MPs vote to support an entirely appointed second chamber.

Pressed on the issue, the Livingston MP said: "I think the moment at which any politician announces the conditions under which he will resign he might as well go straight away.

"You've got to be committed to the team and committed to the project and I am very committed to what I am doing."

MPs and peers will vote on a range of seven options - from total election to total appointment - on Tuesday.

While MPs are expected to back proposals to elect 60 per cent of peers, the House of Lords is likely to support the 100 per cent appointed option.

Once the Commons and Lords have made their opinions known, the joint committee headed by Jack Cunningham will publish a second report setting out the proposals for change.