It is claimed that one candidate got advance information on the test from a pal who had already taken it.

Two assessment boards were cancelled this week - sparking uproar among other inspectors hoping to get a promotion - and a probe has been launched.

The controversy centres on the presentation part of the exam - and the claim that statistics and other details about it were passed on.

This has been scrapped - but a written test and interviews will go ahead. Former Flying Squad commander John O'Connor said: "It goes against the grain of what the police are supposed to stand for.

"It's unforgivable that the system has been breached like this." He added that all candidates should take the exam on the same day to avoid "potential unfairness".

A Yard spokeswoman said: "We can confirm that two assessments were cancelled this week owing to a potential incident concerning one area of the Chief Inspector process, specifically a presentation exercise."

Two years ago The Sun told how a Yard inspectors' exam descended into chaos with allegations that some of the 562 hopefuls swapped notes on loo breaks. It cost the London force £200,000 to re-sit.