Many of the gadgets sold as offering "shielding" from mobile phone radiation probably have little effect, according to the leader of a £7.4m government-funded research project into the potential health risks of phones.

Professor Sir William Stewart said: "I think the jury is still out on whether shielding devices are worth it. But we see things in the literature which we think will not be very helpful if people buy them."

Children were also told that text messaging was probably safer than using mobile phones for making voice calls, because the phone emits only a brief burst of radiation to send the message. But they were also warned that "it is probably not a good idea to do it under your desk in your lap".

Professor Stewart was speaking at the launch of 15 new projects designed to find out what effects, if any, the radiation from mobile phones has on adults and, by extension, children.

In May 2000 he published a report that investigated the data then available, concluding that as a precaution children should avoid excessive use of mobile phones, while retailers should not push the phones as "essential" to children.

He expressed annoyance yesterday at reports that some phone retailers had ignored his advice.

"Some shops are still promoting mobiles to children, and I think that shows a lack of responsibility ... when we're unsure whether children are particularly susceptible to mobile phones' effects," he said.

No direct link has been drawn between phones and serious consequences for health, although small studies have shown localised effects such as slight heating of bones and shortened reaction times.

The new research will investigate whether the low-power radio waves emitted by mobile phones have any negative impact on the body.

Dozens of companies are selling gadgets that claim to "block all harmful radiation" from phones – although many are treated with scepticism by scientists.

Professor Laurie Challit, who sits with Professor Stewart on the board of the research project, said: "We did think at one stage that it would be useful to have a kitemark to say if such gadgets reduced exposure. But that hasn't been adopted."

Fresh research might re-investigate hands-free kits. The Consumers' Association and the Government produced results that disagreed – one saying such kits increased the radiation at the head, the other saying they decreased it.

Although 80 projects have initially received funding, none will study the effects of phones on children directly – although they will be included in epidemiological studies.

Professor Michael Rugg, the third member of the Stewart panel, said that only one project, out of the 80 proposed, included children as study subjects.

Professor Stewart defended the decision. "There are a lot of ethical issues," he said. "I wouldn't be happy for that to be done. Epidemiology can use children, but direct experiments of the effects on children's brains wouldn't get funding."