Mr Alexander welcomed yesterday’s disclosure in The Independent that many of Britain’s most wealthy people do not seek to avoid paying tax. “We are not saying that all of the super-rich are tax dodgers,” said a Treasury source.

But the Treasury minister will reassure Liberal Democrat delegates: “Fair taxes in tough times mean everyone playing by the same rule book. We have this message to the small minority of wealthy people who don’t play by the rules: we are coming to get you and you will pay your fair share.”

Mr Alexander, who will expand the Affluence Unit to a 300-strong body, will promise a new crackdown on people who hide their assets offshore. A 2009 agreement with Liechtenstein, allowing British people with assets there to bring their tax affairs up to date, was expected to raise £1bn. But the HMRC team working on this project will be doubled and could now net £3bn.

George Osborne’s deputy will argue the Government must stick to its economic Plan A but say its deficit-reduction strategy is more flexible than it is given credit for. “There could not be a worse time to argue that we should abandon our plan,” he will tell the party.