Lingering fears over the safety of the triple jab have caused a sharp fall in the numbers of children being protected against measles, mumps and rubella.

In recent years the Portsmouth area has been free of measles, which can be fatal, but with the disease making a comeback across the country and about one in four local children not immunised, health managers fear that an outbreak is possible.

Across the country there have been 1,049 cases of measles since the beginning of the year – the highest number for 13 years – and in the south central NHS region covering from Hampshire to the Thames Valley there have been 76 confirmed cases.

In Portsmouth almost a quarter of youngsters – 24 per cent – have not had both MMR jabs by the time they have started school. Just five years ago only 16 per cent were not properly protected.

Across Hampshire the uptake is slightly higher although it has also fallen in recent years.

Carolyn Watts, public health development manager at Portsmouth Primary Care Trust, said: 'Measles is a very infectious disease so it is important that a large number of the population is vaccinated to prevent it spreading.

'If vaccine levels drop there is a risk of epidemic with lots of unvaccinated people catching the disease, and for some of them there could be very serious consequences.'

She added: 'The introduction of the MMR jab 20 years ago helped us bring levels of measles infection down to very low levels in the UK. It is worrying to see them climbing again now because up to one in four children isn't getting the vaccine.'

Normally the first dose of the jab is given to children between their first and second birthdays, and the booster dose before they are five.

It is possible to get the three components separately, but not on the NHS. Public health workers advise against this because it extends the amount of time children are left without immunisation.

Anyone wanting advice on vaccinations should speak to their GP or practice nurse.