The outbreak of whooping cough in England and Wales has spread to very young babies who are most at risk of severe complications and death, the Health Protection Agency has warned.
There were another 675 cases in June bringing the total to 2,466 for 2012 so far.
At this stage last year there had only been 311 cases.
Increased levels of whooping cough have also been reported in Northern Ireland and Scotland.
The main symptoms are severe coughing fits which are accompanied by a "whoop" sound as children gasp for breath.
Surges in the number of whooping-cough cases are seen every three to four years. This latest outbreak began at the end of 2011.
Before routine vaccination in 1957, whooping cough outbreaks in the UK were on a huge scale. It could affect up to 150,000 people and kill 300 in one year.
There have been 186 cases reported in infants under three months this year compared to 72 in the same period last year. Five babies have died from the infection.
Dr Mary Ramsay, the head of immunisation at the Health Protection Agency, said she was "very concerned" with the increase in cases.
She said: "Whooping cough can be a very serious illness, especially in the very young. In older people it can be unpleasant but does not usually lead to serious complications.
"Anyone showing signs and symptoms, which include severe coughing fits accompanied by the characteristic 'whoop' sound in young children, but as a prolonged cough in older children and adults, should visit their GP."
In the UK, the whooping cough vaccine is given to babies after two, three and four months. A booster dose is given just before primary school.
Babies are not fully protected until the third jab. It is in the following years that protection is at its peak then it gradually fades. It means you can get whooping cough as an adult even if you had the infection or the jabs as a child.
The Department of Health's Joint Committee of Vaccination and Immunisation is considering ways to tackle the outbreak, such as giving teenagers or pregnant women a booster jab.
Vaccinations for medics working with young babies have already been recommended to protect them and prevent them from spreading the infection.
Figures for the end of March showed 27 confirmed cases in Northern Ireland, compared to 13 in the whole of 2011. At the end of March there had been 150 cases reported in Scotland compared to 22 in the first three months of 2011.
Prof Adam Finn, from University of Bristol, said: "The current vaccination programme has reduced whooping cough in children, but also pushed it back into older age groups.
"Immunity due to vaccine does not last as long as immunity due to infection so as the number of people who have had whooping cough in the past falls, population immunity falls and rates go up.
"This is happening everywhere, not just in the UK."