Scotland Yard said it is "actively recruiting new members of staff"

Nine detectives from the unit are being transferred to other departments.

However the move has come under attack from critics who say it will leave the unit lacking the experienced staff it desperately needs.

The unit tracks down Britain's worst child sex offenders and their work involves looking at images of children being tortured, humiliated and sexually abused.

Concerns have been raised about the welfare of officers who have to carry out these tasks routinely.

In future, service will be limited to three years.

A spokesman for the Metropolitan Police's paedophile unit, said: "The Met has a responsibility towards its officers working in a specialist and challenging environment.

"As such, we need to be mindful of the impact the nature of the work has on their health."

The problem has increased particularly because of the involvement of internet crime Mike Hames Former head paedophile unit Unit 'should be kept intact'

But concerns have been expressed that "rotating" highly-experienced detectives working from the elite unit will cripple the search for dangerous child abusers.

Training a new officer takes at least 18 months.

And the unit is already under-resourced and struggling to face thousands of leads, according to former senior officers.

Mike Hames, who headed the unit for five years until 1994, said he had never seen any evidence officers were adversely affected by their work there.

"Something that may have been overlooked by Scotland Yard is the fact that civilian staff who actually watch this stuff all day every day, have been there 15 years - so the logic escapes me," he told BBC Radio 4's Today programme.

He said the unit was a "centre of excellence" facing a growing crime which needed an injection of officers to cope with it.

"The problem has increased particularly because of the involvement of internet crime.

"But there has been no increase in the number of staff, indeed it has been allowed to wither on the vine.

"Quite frankly it's outrageous. It's unacceptable."

'Actively recruiting'

The unit's undercover work led to the jailing this month of trainee teacher Luke Sadowski who tried to buy a nine-year-old girl for sex on the internet.

Experts have said the personal effect on officers who are responsible for hunting paedophiles should not be under-estimated.

A spokeswoman for Scotland Yard said: "The length of time officers generally spend attached to the paedophile unit is currently being reviewed due to the nature of the work they undertake."

Scotland Yard said it was now "actively recruiting new members of staff" which was part of a "long term strategy to ensure reliance in the protection of children".

The Metropolitan Police's paedophile unit is part of the force's Child Protection Command, a department of 420 officers.