Top earners are already seeking advice on how to defer income, bonuses and dividends until 2013 in anticipation of George Osborne's Budget on Wednesday.
Instead of deferring the measure, the Chancellor should reduce the top rate to 45p immediately - or risk losing tax revenues, incurring higher borrowing costs and compounding the 50p tax fiasco, the accountants warned.
Ian Gorham, chief executive of Hargreaves Lansdown, told The Daily Telegraph: "The Chancellor needs to be decisive and any tax cuts should take effect immediately. Announcing a tax cut that only starts in April 2013 is almost certainly a bad idea.
"Many 50p tax payers could perfectly legally defer income – for example, decide not to take dividends, bonuses or salaries from their businesses – for a year to wait for the lower rate. The Chancellor would lose substantial tax revenue in 2012, which would need to be replaced by borrowing. The economy would also miss out on substantial spending and investment; 50p tax payers are big spenders and investors – so that could hurt the recovery."
There are concerns that top earners may seek to delay taking income for even longer if the Chancellor confirms plans to reduce the top rate to 40p by the next election.