The coalition is facing its sternest test of unity as MPs start debating controversial changes to the House of Lords.

Penny Mordaunt MP, one of the Conservative rebels, says she is not against reform but believes "this is really a dog's breakfast of a bill".

"You can't reform the House of Lords without having an effect on the House of Commons," she told the Today programme.

"At best, you would get gridlock between the two chambers.... and at worst you would have a constitutional crisis."

Mark Harper MP, minister for constitutional change in the Cabinet Office, defends the scheme saying it would make Parliament "a whole lot stronger and it would hold the government to account better".

"That's a very strong conservative position," he told James Naughtie.

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