Alistair Darling will face the latest tricky engagement of what has been a tough few months when he addresses business leaders.
David Cameron has vowed to fight plans to raise capital gains tax (CGT) and called for a simpler tax regime.
On Monday, Gordon Brown told the CBI conference that he would continue to "listen and discuss" the thorny issue of capital gains tax reform.
Speaking at the CBI conference, the Conservative party leader said he would fight plans to raise the tax from 10% to 18% "every step of the way".
Mr Darling plans reforms to the tax that will lead to some people's bills rising from 10% of gains to 18%.
Mr Cameron demanded chancellor Alistair Darling abandon the plans that were confusing and would hit entrepreneurs.
The CBI says that will penalise small businesses and entrepreneurs.
Mr Darling is expected to face tough questions from business leaders when he speaks later.
The simplification of the capital gains tax system was supposed to allow the Treasury to raise more money from the very wealthy people who run private equity firms.
'Sticking plaster'
Last year Mr Cameron went to Iraq instead of the CBI
The simplification of the CGT system was meant to allow the Treasury to raise more money from the very wealthy people who run private equity firms.
Conservative leader David Cameron will raise the heat on Mr Darling when he addresses the same conference.
But the CBI among others have attacked the plans saying they would act as a brake on business and discourage new firms from starting in the UK.
He is expected to call on the chancellor to admit he has made a mistake and abandon or radically alter his plans.
Mr Cameron said that Mr Darling "must tell us what the capital gains tax system will look like in five months time".
He will say that confusion about the government's intentions has left entrepreneurs and investors in the dark.
But Mr Cameron also needs to go on a charm offensive with CBI members, many of whom were offended this time last year when he cancelled his engagement to speak at their conference to visit troops in Iraq.
During the first day of the event on Monday Gordon Brown said he would "listen and discuss" the thorny issue of CGT reform.
Mr Darling's speech comes as the Labour Party investigates who knew that David Abrahams, a property developer from the North East of England, had donated almost £600,000 to the party through associates.
"If the chancellor attempts further changes, in order to recover the situation, they must not be sticking plaster changes to proposals that have so angered entrepreneurs," said Mr Cameron.
Mr Brown is sure to face questions about that at his monthly news conference, as well as the news revealed by Mr Darling last week that HM Revenue and Customs had lost the records of millions of Child Benefit claimants.
"If the chancellor fails to abandon or radically alter his approach, we will fight this tax hike on Britain's entrepreneurs every step of the way, inside and outside parliament."
In addition, despite the naming of Virgin as the preferred bidder for Northern Rock, there is no sign that the government's loans to the troubled bank will be paid back in full soon.
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