Concessions to sweeten the changes in capital gains tax announced in last month’s Pre-Budget Report will be unveiled by Alistair Darling, the Chancellor, before Christmas.

Mr Darling will appear before the annual conference of the CBI today and endeavour to reassure members that he is considering the representations that they and other industry groups have made.

However, the changes, including plans to give substantial tax relief to small businessmen who sell up and retire, will be set out to Parliament first, Treasury sources said last night.

That concession, first suggested by the Federation of Small Businesses (FSB), is one that the Government is believed to have already accepted.

John Cridland, deputy director-general of the CBI, said: “If he [Mr Darling] just does what the FSB want him to do, we will say ‘Thank you’, but it isn’t enough. The clock is ticking. Leave it much longer and the damage will be done. Owners are making their own judgment, the trust has gone and people will simply exit if they can make a viable sale.”

Gordon Brown told the conference yesterday that Mr Darling was involved in discussions about the “implementation” of the new regime, raising speculation that there may be longer transition periods for people affected by the decision to bring in a flat rate of capital gains tax at 18 per cent.

The Chancellor will say that the Government is determined to stick to its long-term course and take the tough decisions necessary to maintain stability: “This Government’s determination to take the tough decisions means that even in today’s uncertain times, with turbulence in international financial markets and record oil prices, we can be confident of the resilience of the UK economy.”

The Chancellor is also expected to defend his role in the rescue of Northern Rock. He will say: “I believe it was right to intervene. That it was right to put in place guarantee arrangements for savers. And, above all, it is right to see it through.” He will argue that it would have been far worse to have allowed the bank to fail.