Tax experts had predicted that, in his Budget speech, Alistair Darling would increase CGT from 18% to close the gap between it and the income tax rate of 50% for those earning a salary higher than 150,000.

Instead the chancellor intends to keep the tax at its current rate, and instead increase entrepreneurs' levels of relief on CGT from £1 million to £2 million.

Richard Mannion, national tax director at Smith and Williamson, told HR magazine: "CGT can't stay at 18% for long - it is unsustainable and the Government will have to make changes sooner or later.

"There are people in darkened rooms talking about how high earners can change their earnings into capital gains and save 32% worth of tax. Not too many people pay CGT so it won't be a vote loser and a change in the tax regime is inevitable in the not so distant future."

Darling set the rate of CGT at 18% himself in 2008, and seemingly did not want to perform a U-turn so close to a general election.

The news comes as he confirmed the 50p tax rate would come into effect on 6 April but discussed further anti-avoidance measures to prevent people finding ways not to pay tax. These include tax avoidance deals with Lichtenstein, Grenada, the Dominican Republic and Belize.

The chancellor added that the 50% tax on bank bonuses implemented in the pre-budget report had raised twice as much revenue as expected - £2 billion.

He said 60% of the tax increases would only affect high earners and added this was not part of Labour's "dogma or ideology".