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There are weaknesses in the way the performance of locum doctors is checked in the UK, a review suggests.

Doctors have had to take part in five-yearly "MOTs" since 2012 to ensure they are fit to continue working.

The General Medical Council said the system was working well, but highlighted problems with the way it was being applied to locums.

The review by the regulator found hospitals were "unwilling to provide frank feedback" on locums.

Sometimes hospitals just told agencies "please don't send this doctor again" rather than giving full reasons.

Locum agencies are also failing to carry out regular appraisals needed to effectively judge whether licences should be renewed.

The GMC said it made assessing the performance of locums more difficult - they are twice as likely to have applications for revalidation deferred while more evidence is sought than other doctors.

But the review - led by Sir Keith Pearson, chairman of Health Education England - praised the way the system had changed the medical profession.

Until 2012 there had been no regular checks on performance. Doctors only lost their licence if serious concerns were raised and they went through the GMC's fitness to practice system, the disciplinary process for doctors.

How the checks work

Revalidation takes place every five years. So far 205,000 of the 230,000 doctors on the medical register have been through the process

A decision on whether or not to allow a doctor to continue to practise is based on annual appraisals and feedback from patients and colleagues

It is up to responsible officers in each organisation, normally medical directors, to make a recommendation

Minor issues lead to revalidation being deferred for a short period while the problems are addressed. Nearly 40,000 doctors have found themselves in this position

Serious concerns lead to the doctor being referred into the GMC's disciplinary process

If a doctor does not engage with the process they lose their licence - this has happened in nearly 3,500 cases

The introduction of regular checks - dubbed a medical MOT - had been talked about for more than 30 years.

But serious consideration started to be given following scandals like that of the family GP and serial killer Harold Shipman and the deaths of the Bristol heart babies.

While the system was generally working well for doctors other than locums, the GMC did raise concerns about the approach to patient feedback.

It is considered a key part of the revalidation process, but doctors were not always continually seeking feedback.

Health Minister Phillip Dunne said the revalidation process was proving to be a vital process, but said he wanted to see progress on the way it worked for locums.

"We know there is more to do to ensure that revalidation is as streamlined and effective as possible for doctors," he added.