City firms are drawing up schemes to help staff to dodge the new 50% rate of income tax, ahead of what is expected to be a rapid return to bumper bonuses in the Square Mile.

Grant Thornton, the accountancy firm, has sent a document to clients outlining one scheme that could afford a “potential tax saving of approximately 40%”.

This week Alistair Darling is expected to fire a shot across the bows of the big banks by warning they should curb bonuses. Treasury officials say pay will be a key element of Darling’s white paper on regulation, published on Wednesday.

The chancellor has ruled out direct controls, but has accepted a recommendation from Lord Turner, chairman of the Financial Services Authority (FSA), that remuneration should form part of the regulatory risk assessment for banks and other institutions.

The FSA will examine whether remuneration policies, including bonuses, encourage “excessive risk taking”. Banks that have such policies will either be required to abandon them or face higher capital requirements and more intrusive regulation.

The Grant Thornton scheme, one of several being sold to City firms, is called a “growth securities ownership plan”. Individuals would pay only capital gains tax at 18% on any profits. Darling recently introduced a new top rate of income tax, which will apply to earnings of more than £150,000 a year from next April.

Mike Warburton, senior tax partner at Grant Thornton, said it was designed to help clients “incentivise employees, and it is not in any way related to the 50% income tax rate”.

Other companies are said to be examining more exotic ways around the 50% tax levy. They include paying the bonus through financial derivatives called “contracts for difference”. These are attractive because they have no settlement date, meaning the individual can cash out whenever they like.

These exotic schemes are similar to the practices adopted by companies in the 1980s when they gifted employees with valuable items such as gold coins and fine wines in order to avoid high taxes. The government eventually clamped down on these methods, and tax experts warn that the same will happen now.

John Whiting, a tax policy director at the Chartered Institute of Taxation, said: “The paymaster-general has said that if companies try schemes to avoid taxes, they will take action, even retrospectively.”

Big bonuses are expected to make a return in the City, but only at some institutions and in certain groups of staff. The investment bank Goldman Sachs is expected to announce a total compensation pot of about $20 billion (£12 billion).

Matthew Oakeshott, Liberal Democrat Treasury spokesman, said: “Paying big bonuses in a deep recession is offensive enough. But it is far worse to disguise them as capital gains so fat cats pay less tax than their staff. Has this abusive tax avoidance scheme been properly disclosed to the Treasury? HMRC [HM Revenue & Customs] must ban these bonuses in dark glasses now.”

Separately, both of the state-controlled banks — Royal Bank of Scotland and Lloyds Banking Group — have bonus schemes in place that could once again pay out millions to staff early next year.

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