The average pay packets of FTSE 100 bosses has grown by around £500,000 leading to an "unhealthy and growing" gap in wages, according to a new report.

The average remuneration for a FTSE 100 chief executive was £5.48 million in 2015, up from £4.96 million in 2014.

Independent think tank High Pay Centre said its research also found chief executives were paid on average 140 times more than their employees, adding that only a quarter of FTSE100 firms were accredited by the Living Wage Foundation for paying the voluntary living wage to staff.

This is currently £8.25 per hour outside of the capital and £9.40 per hour in London, higher than the Government's obligatory minimum wage rates.

West Midlands firms currently on the FTSE 100 include utilities National Grid and Severn Trent and builders' merchant Wolseley.

High Pay Centre's report also said the top ten highest paid chief executives in both 2014 and 2015 were all men, including Sir Martin Sorrell of WPP, Bob Dudley of BP and Antonio Horta Osorio of Lloyds Banking Group.

Only one company had an employee representative on the board, while none published details of the pay ratio between chief executives and other employees, the report added.

Stefan Stern, director of the High Pay Centre, said: "There is apparently no end yet in sight to the rise and rise of FTSE100 chief executive pay packages.

"In spite of the occasional flurry from more active shareholders, boards continue to award ever larger amounts of pay to their most senior executives.

"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.

"In particular, we support two of her 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 the voice of the ordinary employee must be heard in discussions over executive pay."