Return of the nit: We've got the lousy weather to thank, and many of the treatments don't work

Call the nit nurse! A miserable summer followed by a long winter have created an explosion of head lice infestations across Britain – and the country's nits are more resilient than ever to treatment.

More than half of all four- to 11-year-olds catch head lice every year, and there are signs that the last 12 months have been particularly bad as poor weather has driven children to spend more time indoors.

In the face of the epidemic Britons are spending more than ever on head lice treatments. Last year they spent nearly £23m, according to the market research firm Nielsen. However, experts are warning that too many commercial products are hopelessly inadequate in the face of the nit onslaught.

A study of insecticide-based shampoos that use the chemical permethrin found it is only 15 per cent effective in getting rid of infestations, down from a rate of almost 100 per cent when it was first used widely in Britain two decades ago.

The research, which will be published next month in the medical journal BMC Dermatology, was conducted by Ian Burgess, director of the Medical Entomology Centre. He said: "Britain is the only place in the world that I know of where you have quite such a level of resistance to insecticides."

Mr Burgess said that growing levels of insecticide resistance, together with poor weather conditions, are causing the number of nit infestations to spiral. "There are indications that during the last year there was an increase in infestations compared to the year before, because we had such a terrible summer. Last summer was a bonanza for nits because people were indoors and they were spreading around. It's about being in close proximity for longer periods of time. If you're inside watching telly or playing video games you're closer than if you're outside kicking around a football."

Previous research by public health workers in Wales found that eight out of 10 products to treat infestations did not work. The situation is so bad that 88 per cent of parents said they wanted the nit nurse to come back, when asked in a survey by the parenting website Netmums.

Dee Wright runs Hairforce, a company that tackles head lice by sending in "lice assassins" to vacuum and blast hair with hot air. They have seen the number of appointments soar since 2011, killing millions of head lice on 2,400 clients in the last year – double the number in 2011. Ms Wright said: "We're seeing a lot more people, and with the cold weather we're seeing a lot of heavy infestations. Lots of central heating, lots of hats and being indoors – head lice adore all of that incubation."

It took Maxine Kelly, 37, from Northampton, three months to rid herself and her two children, George, 15, and Hattie, four, of lice. "Last year head lice treatment cost me over £70 as it just was not working," she said. "By the time it reached the kids it had reached me too. We bought four lots of chemical treatment, which cost upwards of £16.99, for all three of us. I kept doing the treatments and then they came back again. In all it took us three months to be rid of them, and the most effective thing in the end was a £2.99 bottle of conditioner and a nit comb."