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The ex-housebuilding executive, Jeff Fairburn - who gave up part of his huge bonus after a public outcry - is facing criticism over the shares he gave back.

The former Persimmon boss said he would forego half his shares - but analysis carried out for the BBC found the ones he retained were the most beneficial.

The reduction brought the value of his bonus down from £100m to £75m.

Although the method used was legal, a more arbitrary cut across all the stock could have brought him down to £60m.

Last week, Mr Fairburn was forced out due to "distraction" over his deal.

The bonus award was one of the biggest in corporate history and attracted additional controversy because the housebuilding firm's performance has benefitted from low interest rates and the government house-buying incentive scheme Help to Buy.

At the firm's annual meeting in April, 48.5% of investors voted against the remuneration report - although 85% had approved the pay scheme when it was unveiled in 2012.

Mr Fairburn's bonus - and that of other senior executives at Persimmon - was based on awards of shares in a long-term incentive plan (LTIP).

The shares were awarded to Mr Fairburn in three tranches, which in turn required him to pay different amounts towards the cost of the shares.

One of the tranches required him to pay nothing, another almost £4 and another almost £10.

When Mr Fairburn agreed to halve the number of the shares he had been awarded, it was done on the basis of cancelling the tranche which cost him most to obtain the shares first.

Although there was nothing improper about the method used to reduce his shareholding, if the 50% share reduction had taken place equally across the three tranches of shares it would have cost him more to buy the shares and further cut the value of the share awards.

The method that was used reduced the total number of shares he was entitled to by 50%, as was announced by the company.

The value of the bonus moves with the share price and the BBC calculations are based on rounded estimates.

Both Persimmon and Jeff Fairburn declined to comment on the BBC's calculations.

Media playback is unsupported on your device Media caption Last year Persimmon's previous chief executive refused to answer questions about his pay

Luke Hildyard, director of the High Pay Centre, said it seemed like a "crafty way" to reduce the bonus.

When Mr Fairburn's departure was announced, the housebuilder said the bonus issue was having a "negative impact" on the firm's reputation and on "Jeff's ability to continue in his role".

Last month, Mr Fairburn walked away from a BBC interview when asked about his pay.

"I'd rather not talk about that," he told a BBC reporter when asked in October about the pay award.

Mr Fairburn had also attempted to defuse the row by saying he would give a substantial part of the bonus to charity. He has not said how much he will donate.

Mr Fairburn has not yet sold any shares so does not immediately have those funds to donate.

When his departure was announced, Mr Fairburn said that he had hoped the plans to create a charitable trust and waive part of the share award would allow the company to "put the issue... behind it".

"However, this has not been the case and so it is clearly now in the best interests of Persimmon that I should step down," he said at the time.