Exclusive: Former A4e boss Emma Harrison was one of those who took advantage of a lag between the announcement and the start of the 50p top tax rate, Channel 4 News learns.

The analysis of the gains from the 50p tax rate could now be called into question, as Channel 4 News reveals it could be skewed by entirely legitimate tax avoidance strategies.

The 50p top rate of tax for people earning over £150,000 was announced by Alistair Darling in 2009, but it didn’t come into effect until 2010. This left an opportunity for dividends to be paid in this period that would not be subject to the top tax rate.

George Osborne has asked Her Majesty’s Revenue and Customs to establish once and for all how much money the 50p rate raises. But Channel 4 News can reveal how it is almost impossible for the HMRC to come up with an answer for the chancellor – because of a series of anomalies in the way Britain’s richest have paid themselves.

On Wednesday when the chancellor announces the phasing out of the 50p rate he will publish figures that show the tax raised a pretty modest amount of money – less than a billion pounds. But the data (and therefore the argument for dropping the tax) could be very misleading – because the 50p rate has only been in force for a very small time and many people used these perfectly legal methods to reduce their tax liabilities by changing the date of their company dividends payment.

An independent estimate for Channel 4 News suggests that up to £20bn may have been paid pre-emptively, which would otherwise have generated £2bn for the exchequer.

Channel 4 News Economics Editor Faisal Islam has discovered that one of the people who benefited was Emma Harrison, the Conservative adviser whose “welfare to work” delivery company A4e is now facing various investigations into how it spent public money.

There was controversy in January when it emerged she had received a £7m dividend from the company. But now we can reveal that the timing of this payment means that Harrison avoided nearly £800,000 pounds in income tax as well.

Over the last five years, A4e has paid out dividends: 85 per cent of which go to Emma Harrison, reflecting her shareholding. Then in 2011, a dividend of just over £10m was paid, four times larger than any previous year.

Before 2011, the first part of the annual dividend was paid after the start of the tax year.

But with the 2011 dividend, the largest part – £8.5m – was paid on April 1st 2010, before the new tax year. That shift in timing meant the dividend was taxed at the old 40 per cent top rate.

Because of the way dividend income is tax, this saved Emma Harrison £803,000 in tax on her share.

This is an entirely legitimate strategy. However, it does mean that the analysis on the gains from the 50p tax rate will not be based on entirely accurate figures. Assuming others took advantage of this opportunity, the figures will only reflect one complete year under the new top tax rate.