The Health Protection Agency carried out research amid concerns that many parents are not immunising their children.

Research linking MMR to autism has been widely discredited in medical circles.

The latest annual figures showed just 62% of toddlers in south-east London, had been immunised. Health experts say the figure should be 95%.

The World Health Organization says that is the level needed to provide "herd immunity", protecting everyone against the diseases.

Critical watermark

The medical newspaper Pulse has warned that there could be a measles epidemic this winter on a scale last seen in the 1960s.

It said that lowering levels of immunity meant as many as 12% of children and 20% of adults could be hospitalised if infected by measles.

Using its data, the HPA calculated that the "reproduction number" - the number of infections resulting from each case - had already reached the critical watermark of 1.0 in London.

Dr Mary Ramsay, consultant epidemiologist at the HPA, said: "We're predicting an epidemic from this, and many places in London are already at a point where an epidemic can occur."

Small outbreaks

The HPA said it was working with health officials in London to formulate an emergency response plan to stop measles spreading.

Uptake fell to 69% in north-west London and 62% in south-east London, according to official figures for the year April 2003 to March 2004, and some parts of London have already seen small outbreaks of the infection.

Phil Johnson, editor of Pulse, said: "We need 95% coverage among young children to be sure of preventing epidemics and in some parts of London uptake of MMR is as low as 65%."

A Department of Health spokeswoman said: "There is no measles epidemic in the UK - so far this year the cases of measles are less than they were last year."

She said more recent statistics suggested coverage was rising.

But she added: "However we to recognise that overall coverage is lower than ideal and this leaves a concerning vulnerability."