Pay for the chief executives of the UK's biggest 100 companies has fallen by 17% over the past year, as political pressure on FTSE 100 firms to justify the salaries of their highest earners has intensified.

The average pay of FTSE 100 bosses dropped to £4.5m last year from £5.5m during the previous 12 months, according to data compiled by the High Pay Centre, a think-tank that campaigns against excessive corporate pay. The decline reverses a two-decade climb in salaries for the top-paid CEOs.