Liberal Democrat leader Nick Clegg has been hit by the resignation of three frontbench MPs.

Justice spokesman David Heath, Scotland and Northern Ireland spokesman Alistair Carmichael and countryside spokesman Tim Farron all quit in order to support a referendum on the EU Reform Treaty.

The three were among 13 Lib Dem MPs to defy the whip and back a poll on Wednesday.

Clegg had ordered his MPs to abstain on a Conservative proposal for a referendum.

But Heath said earlier that he would defy the whip and vote with the Tories along with up to 14 party colleagues. 

Farron also told the BBC he would quit his frontbench role and back a referendum.

Clegg said he "greatly regretted" the loss of the three spokesmen.

"Though we have disagreed on this issue I fully understand and respect their strongly held views on the subject," he said. 

"However, as they have recognised, the shadow cabinet cannot operate effectively unless the principle of collective responsibility is maintained.

"I am grateful for their offer of strong support from the backbenches, and I know that they will all play a very significant role in the future of our party."

However Heath said: "I owe it to my constituents to be consistent on this matter. I am quite clear that an in-out referendum would be a better course of action.

"But if I'm asked whether I support a referendum on the much narrower question [of the Lisbon treaty], my answer has to be yes."

Earlier this week Carmichael told ePolitix.com: "If we proceed on the basis that we are all acting according to our own lights and with good will and good faith, then whatever the outcome is at the end of the day, I am fairly hopeful that whatever differences we may have as a party we will still come through it as a fairly united force."

Clegg immediately moved to announce who would be taking over their responsibilities, with environment spokesman Steve Webb to speak on the countryside, home affairs spokesman Chris Huhne's on justice and international development spokesman Michael Moore on Scotland and Northern Ireland.