The government's chief medical expert has defended the controversial MMR vaccine as a safe and effective way to protect children from disease.

Professor Sir Liam Donaldson, the Chief Medical Officer for England, warned that switching to a single measles jab would inevitably put more children at risk.

Professor Donaldson's comments came as public health officials confirmed that they were investigating a new outbreak of measles in Barnet, north London.

If we went to a single vaccination programme we would have to accept the certainty of more cases of these diseases

Professor Sir Liam Donaldson

One confirmed case and five suspected cases have been linked to a school in the town.

Meanwhile, a surge in public demand has led to supplies of the single measles jab running out across the UK.

And the BBC has uncovered disturbing evidence that some doctors are bullying parents into giving their children the combined vaccine.

Russian roulette

The MMR vaccine has been linked to autism and bowel disorders. However, the medical establishment is adamant that such a link does not exist.

Professor Sir Liam Donaldson warned about abandoning MMR

Professor Donaldson said that latest research which suggested the measles virus was found in the guts of autistic children was "riddled with flaws".

He added that MMR was able to protect children against three separate serious diseases of childhood in a combined jab.

He said: "Splitting the vaccines into separate individual doses would mean six separate jabs spread over a considerable period of time.

"This would leave a pool of children unprotected and create the conditions where there would a resurgence of not just measles, but mumps and rubella as well with all the serious complications that attend those diseases.

"If we went to a single vaccination programme we would have to accept the certainty of more cases of these diseases, and we would be doing so on the basis of no sound scientific evidence, indeed we would be doing so in the face of major evidence in favour of MMR."

"We would literally be playing Russian roulette with our children's health, and that would be an irresponsible and unacceptable way to run a programme which is meant to be protecting children's health."

Before MMR was introduced 14 people died of measles a year in England and Wales.

Parents being bullied

Patients tell me that their GPs have threatened to remove them from their lists if they do not give their child the MMR

Dr Richard Halvorsen

An investigation by the BBC Today programme has found that parents are being bullied into accepting the MMR vaccine by doctors.

A mother from East Sussex told the programme that she had been threatened by her new GP.

"He said some GPs would consider not to have children vaccinated a form of child abuse, and he said children could even be made a ward of court."

Dr Richard Halvorsen, of the Holborn Medical Centre in London, said: "Patients tell me that their GPs have threatened to remove them from their lists if they do not give their child the MMR.

"They are being bullied, pressurised and patronised into giving their children the MMR vaccine.

Doctors receive an extra payment if they met targets for immunising a high proportion of patients.

Mike Stone, chief executive of the Patients Association, said: "There is a huge amount of patients who have been told that unless they have the MMR injection they are very likely to be struck off their doctor's list. That is totally unacceptable."

Dr Andrew Wakefield, the scientist whose disputed research raised doubts about the safety of MMR, is standing by his claims.

He said: "There should be no discussion of measles epidemics, of deaths, because we have an alternative to the MMR.

"It is unsustainable on the part of the Department of Health not to offer a choice."

Dr Wakefield is helping to prepare a legal challenge by eight families who suspect MMR is responsible for their children's autism.

The UK's only approved importer of the single vaccines told doctors on Wednesday night that it had run out of supplies. It will be six weeks before supplies can be replenished.

New cases

So far 11 cases of measles have been confirmed among children in Streatham, south London, where take-up levels of the vaccine are among the lowest in the UK. Tests are being carried out on 18 suspected cases.

None of the infected children had received the MMR jab.

Four cases have also been detected in the Gateshead and South Tyneside area.

The latest figures show UK-wide take-up of the MMR jab fell to a record low between July and September last year with a coverage rate of just 84.2%. The recommended rate is 95%.