LONDON (REUTERS) - The average pay of bosses in Britain's FTSE 100 rose more than 10 per cent in 2015 to an average of 5.5 million pounds, according to a survey that is likely to reinforce a drive by Prime Minister Theresa May to curb excessive pay.

Britain's new prime minister has denounced as irrational and unhealthy the yawning gap between the amounts paid to bosses and those paid to the average worker, vowing to better align incentives with the long-term interests of companies.

A survey by the High Pay Centre said the average FTSE 100 chief executive's pay package hit 5.48 million pounds (S$9.62 million) in 2015, up from 4.96 million pounds in 2014, meaning CEOs now earn 140 times more than their employees on average.

The survey said FTSE CEO pay had now risen by 33 per cent since 2010.

"There is apparently no end yet in sight to the rise and rise of FTSE 100 CEO pay packages," director Stefan Stern said. "The High Pay Centre was delighted by Theresa May's recent intervention on this issue. There now seems to be political will and momentum behind attempts to reform top pay."

The country has seen a resurgence in investor activism in the last year, with several FTSE 100 bosses criticised at annual general meetings for taking ever larger pay deals at a time of weak economic growth.

Ms May, who became prime minister in July, used a speech before she took office to set out her plans for the economy, arguing that it did not work for everyone in society and needed to be reformed.

Her proposals included making shareholder votes on corporate pay binding. She also said she wanted to see more transparency around bonus targets and the publication of the ratio between a CEO's pay and that of the average company worker.

According to the survey, the three highest paid bosses in the blue-chip index were WPP's Martin Sorrell, Tony Pidgeley at housebuilder Berkeley and Rakesh Kapoor at Reckitt Benckiser.

The 70 million-pound pay package awarded to Mr Sorrell was one of the biggest payouts in British history.

A third of investors refused to back the deal while Mr Sorrell, one of the best known businessmen in Britain who built the advertising group from scratch, said all his wealth and interests were tied up in the future of the company.

The High Pay Centre is an independent think tank that monitors executive pay and campaigns to reduce the income gap.