West London Mental Health Trust, which runs the hospital, rebuffed allegations that it was £4m over its capital estates and facilities budget.

However, it had identified an overspend of £3.9m on the capital budget for the whole trust early in the financial year, not just for the hospital.

In the statement, the trust said the allegations of overspending at the hospital were “categorically not the case”.

It said: “That investigation is seeking to determine if there were projects that in fact overspent, the scale of any overspends, and whether any of the projects were continued despite being unfunded and/or unauthorised and if so, how that occurred.

“We are unable to comment in detail on the investigation while it is on-going.

“However, we can assure the public the trust takes its duty to spend taxpayer money on priorities that make a difference to patient care very seriously.

“The investigation will be full and impartial, and will interview all those involved in the management of the department at that time.

“The trust board will take any appropriate actions as recommended in the final report.” Broadmoor Hospital opened in 1863 as England’s first asylum for the criminally insane and remains as one of only three high-security mental health hospitals in the country after becoming a hospital in 1948.

The hospital, which treats some of the country’s most dangerous offenders, is currently undergoing a £242m redevelopment, with a new 16-ward, 234-bed unit expected to be built by 2017.

Despite this, independent investigators are looking into allegations of fraudulent activity at other facilities operated by the trust.