Three babies died of whooping cough in England and Wales in October in the biggest outbreak for 20 years, health officials said Friday.

The resurgence of the highly contagious bacterial infection has surprised doctors, a report on the issue from the Health Protection Agency (HPA) said.

There are concerns the real number of cases could be much higher, with many children so far undiagnosed.

The total number of babies under the age of three months who have died so far this year stands at 13, according to the HPA.

A total of 7,728 cases of whooping cough were confirmed in the first 10 months of 2012, the agency said.

The disease recurs in Britain every three to four years but this figure is seven times higher than the overall number of cases in 2011 -- 1,118.

In a move to combat the outbreak, all women between 28 and 38 weeks pregnant are being offered a vaccination.

Dr Gayatri Amirthalingam, consultant epidemiologist for immunisation at the HPA, recommended all pregnant women should take up the offer and added: "Parents should also ensure their children are vaccinated against whooping cough on time, even babies of women who've had the vaccine in pregnancy -- this is to continue their baby's protection through childhood."