Leaders at the Association for Consultancy and Engineering said the pay delays risk driving firms out of business.

An ACE study showed payment times for smaller consultants had lengthened to 90 days this year up from 78 days last year.

But larger firms saw payment times improve to an average of 70 days – down from 80 days in last year’s study.

Revenue per fee earner among companies with fewer than 50 staff rose 9% to an average £79,100 with a similar figure at larger practices.

ACE chief executive Dr Nelson Ogunshakin said: “These figures show there is a long way to go to ensure clients pay promptly for the work our industry does, even at a time when financial constraints have hit small companies hard.

“However, rising income per fee earner shows how companies are striving to extract the maximum value from the talent the industry has.

“This figure may rise further in the years ahead if our education system doesn’t develop the talent needed to eventually take the baton from an older generation of engineers.”