Hopes in the business community that Alistair Darling will be forced into a rethink of his plans to shake up capital gains tax (CGT) grew last night after signs of Labour backbench unease.

With the Treasury and Downing Street holding firm on the proposal to end taper relief, which means the rate falls to 10 per cent after assets have been held for two years, a senior Labour MP called last night for delay.

The measure to bring in one standard rate of CGT at 18 per cent is due to come into effect in April, although it will have to be voted on in the Commons. It has provoked a hail of opposition from bodies that say it will damage small firms. Any serious Labour opposition would jeopardise the plans.

George Mudie, a former Labour deputy chief whip and leading supporter of Mr Brown, called for the changes to be shelved for a year pending a policy rethink.

Mr Mudie, a member of the Treasury Select Committee, told The Times: “I think that there are unintended consequences to the measure that are becoming apparent. There is no real financial reason why we don’t just consult and do it later – put it off and then make a job of it.”

A one-year delay, bringing in a revised system for capital gains tax in April 2009, would cost the Treasury £350 million, as the full yield from the Chancellor’s plans rises over time.

Some employees who buy shares in their company are among the losers from the proposed changes, along with small business owners who want to sell up and investors who have held assets for a long time - who will suffer twice, once with the loss of taper relief and additionally because no account will be taken of inflation.

Holders of employee shares with a value higher than the annual allowance of £9,200 will see their tax rate rise from 5 to 18 per cent, according to John Whiting, an economist at Price-waterhouseCoopers.

They join private equity businesses, other entrepreneurs and specialist investors in unquoted companies on the alternative investment market (AIM) among the chief losers.

The biggest gainers are second home owners and investors with large shareholdings of quoted companies. Unions have joined the main business organisations in raising concerns over the plan. And with the measure certain to come before the Commons unless Mr Darling backs down, the opposition parties have been looking for Labour MPs to join them.

Meanwhile, George Osborne, the Shadow Chancellor, has said that there is a good chance that Mr Darling will have to back down. He spoke as an independent report commissioned by the Conservative Party found that the changes to CGT will undermine Britain as a place to create and grow new businesses.

Interim results of the study conclude that despite a decade of policy initiatives that have been attempting to promote entrepreneurship, new businesses are struggling to grow in an increasingly uncompetitive tax environment.

The study, produced by the European School of Management, said that the proportion of businesses that achieved an annual turnover above £1 million five years after their creation has nearly halved from 29 per cent in 1998 to 16 per cent in 2006.

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