Nick Clegg makes his keynote speech on Sunday

The Lib Dem leader insisted he had no regrets after 15 of his MPs defied him to vote for an EU treaty referendum.

And he defended a Commons walkout last week saying people could expect more protests against the political "establishment" under his leadership.

He was speaking as the party gathers in Liverpool for its spring conference.

The Lib Dems staged their Commons protest after they were refused permission to table an amendment calling for a referendum on Britain's future in Europe.

The protest - led by the party's foreign affairs spokesman Ed Davey, who was barred from taking further part in the debate, is reported to have been opposed by Mr Clegg's predecessor as Lib Dem leader Sir Menzies Campbell.

'Principled party'

But speaking to reporters in Liverpool, Mr Clegg said he had "no regrets" about the incident.

"You are going to get a lot more of that," said the Lib Dem leader.

"When you have a system that is as sclerotic as this - tied up between the two parties - it is quite right for a principled party such as the Liberal Democrats, which believes in political reform from saying, from time to time, 'hang on, this is an absolute joke'".

Aides said Mr Clegg would set out specific proposals on fighting what he saw as Britain's outdated and hidebound political system in his keynote speech to the conference on Sunday.

They cited Mr Clegg's pledge to go to jail rather than carry an ID card - or treasury spokesman Vince Cable's boycott of an official dinner for the Saudi Royal Family - as examples of what the party leadership had in mind.

'Honest debate'

They also insisted the Lib Dems would continue to campaign for an "in or out vote" on Europe if and when the Lisbon treaty has been ratified. Three Lib Dem MPs quit the frontbench in order to support the failed Tory attempt to force a referendum on the Lisbon treaty.

The Lib Dems insist their position - opposing a referendum on the Lisbon treaty but backing a vote on the wider question of whether Britain should remain in the EU - would provoke the "honest" debate that voters wanted.

But opposition critics have accused the party of pushing for an "in or out" vote to paper over their divisions on the Lisbon treaty.

Mr Clegg said he had known "for months" that he faced a rebellion on the EU treaty referendum but he refused to be drawn on how many of his MPs would have voted for one if they had not been ordered to abstain on a three-line whip.

'Establishment collusion'

He said he had tried to "maximise the amount of unity while at the same time knowing it would not be complete".

Asked why he had not simply given his MPs a free vote, he said Europe was "crucial to the party's identity" and not an "issue of individual conscience".

He said he had spoken to the frontbench rebels who had not quit but had not decided on what sanction, if any, he would take against them.

Labour easily defeated a Conservative amendment calling for a referendum on the Lisbon treaty in Wednesday's vote despite a rebellion by 15 Lib Dem and 29 Labour MPs.

In a speech to activists on Friday evening, Mr Clegg restated his commitment to an "in or out" vote - and tackled critics of his handling of the vote head-on.

"I can take the negative headlines. We've faced down the collusion of the establishment parties at Westminster before and I will not be afraid to do it again," he said at a rally in a side room at the Liverpool arena.