NICK Clegg has slapped down Vince Cable over his colleague's call for more borrowing to boost spending, saying that what many have described as "plan B" is simply a myth.

The Deputy Prime Minister also used his keynote speech to the Liberal Democrat spring conference to rebuff Home Secretary Theresa May's threat to pull the UK out of the European Convention on Human Rights, telling delegates that it will never happen while he is around the Cabinet table.

The Business Secretary had spent the previous few days talking up the idea of borrowing more to boost spending, suggesting the "balance of risk" now favoured it over austerity, telling a conference fringe gathering the Coalition had to "really get moving" in boosting investment in infrastructure.

There have been fractious exchanges between Mr Clegg and Mr Cable, who has been told to tone down his "commentaries", which have given political clout to Labour. One senior source made clear the party leadership was "not in favour of higher borrowing".

Mr Clegg told conference in the Brighton Metropole: "I want to make one thing clear – we will not flinch on the deficit. But to be unflinching is not to be unthinking. And the idea that the choice is between a cruel and unbending plan A and a mythical plan B is simply not the case."

The party leader went on to insist that, despite the constraints, the Government was "straining every sinew to invest every available pound into UK infrastructure".

He said: "We will and must do more to mobilise investment into our long-term infrastructure needs. I agree with that. Vince agrees with that. Danny agrees with that. But, as we all equally acknowledge, there are no cost-free, risk-free ways of finding such huge sums of money."

With the allegations of sexual harassment against the party's former campaigns chief, Lord Rennard, hanging over the hall and the prospect of a jail term today for former Energy Secretary Chris Huhne, Mr Clegg was keen to concentrate on the by-election victory at Eastleigh.

He praised party activists for turning out in their thousands to save the seat for the LibDems, telling them: "I have never seen anything like it ... You were just brilliant."

He said that for the first time in a generation the LibDems could campaign on their record of local and national delivery. "We didn't win in Eastleigh in spite of being in power, we won because we're in power," he argued.

He insisted that the result disproved the theory that the LibDems could not distinguish themselves from the Conservatives in Tory-facing seats.

"The longer you stand side by side with your opponents, the easier your differences are to see. We don't lose our identity by governing with the Conservatives. The comparison helps the British people understand who we are," he argued.

Mr Clegg attacked the Tories head-on when he said: "They, on their own, will never deliver the fairer society; it's just who they are." And he insisted Labour was incapable of "delivering a stronger economy".

His biggest cheer came when he attacked Mrs May for claiming that withdrawal from the European Convention on Human Rights was on the table. "Well, I tell you, it won't be on the Cabinet table so long as I'm sitting around it."

On Friday, Mr Clegg will speak at the Scottish Liberal Democrat spring conference in Dundee.