Image: World Economic Forum/Sebastian Derungs Image: World Economic Forum/Sebastian Derungs

The average pay of the FTSE 100's chief executives rose 10% to £5.5 million in 2015, a new report has revealed.

The High Pay Centre's report also said the median pay for FTSE 100 bosses jumped to just under £4m, 144x that of the average worker on £27,600, which rose just 2% in the same period.

It is further evidence of what Prime Minister Theresa May described as an "irrational, unhealthy and growing gap" between boardroom and shop floor pay. And yet, there is "no end yet in sight", according to Stefan Stern, director of the High Pay Centre.

Speaking to the BBC, Mr Stern said: In particular, we support two of [Theresa May's] main proposals: that companies should be obliged to publish the ratio between the pay of the chief executive and the average worker in the business; and that the voice of the ordinary employee must be heard in discussions over executive pay."

One of the Prime Minister's proposals would see company boards bound to shareholder votes on executive pay. Earlier this year, a string of companies saw their investors vote against plans to increase pay for the top bosses, some of which voted against the proposals in record numbers. However, most of these types of vote are not binding, but merely advisory.

Sir Martin Sorrell, chief executive of advertising giant WPP, earned more than any other FTSE boss. In fact, at £70m, he earned more than double the next highest earners. Tony Pidgley, chief executive of Berkeley Group, and Reckitt Benckiser's Rakesh Kapoor both earned more than £23m in 2015.

The High Pay Centre said there were no female chief executives in the top 10 earners.