London Hundreds of terminally ill children are being denied potentially life-changing cancer drugs because of EU red tape, doctors have claimed.

Doctors have hit out at a European legal loophole that means some pharmaceutical companies only have to test new treatments on adults - even though they could add years to youngsters’ lives.

As the law stands, firms can opt out of testing new treatments on under-18s during clinical trials, meaning that doctors are barred from using them on children as they are cleared for adult use only.

The exemption means children were involved in just half of cancer drug clinical trials over a five-year period, including for treatments which experts say could help them fight rare cancers and brain tumours.

The reason companies ask for a waiver is that they may be running trials for a drug that is unlikely to be safe or effective in children, or to treat a disease that does not occur in children, such as lung cancer.

But doctors say cancer treatments developed for one particular “type” of cancer could also have an effect for other diseases that share the same genetic mutations.

Doctors, politicians and families are now campaigning for the loophole in the EU Paediatric Regulation, which was introduced eight years ago, to be closed.

Cancer doctor Louis Chesler, a consultant at the Royal Marsden Hospital in London, warned that the law was “lagging behind science”.

He said: “Children have died because the legislation did not deliver enough drugs for them. You have to watch kids die because the treatment doesn’t get to them. Removing the waiver would speed the delivery of drugs to children and potentially save lives.”

More than 1,500 children a year in the UK are diagnosed with cancer, and at least 250 die.

Research by the Institute of Cancer Research found that only half of 28 cancer drugs trialled between July 2007 and June 2012 were tested on children. Drugs that have been granted exemption include breast cancer drug Lapatinib and Lenalidomide, developed for a type of bone-marrow cancer called multiple myeloma.

Experts say drugs covered by the exemption could benefit children suffering from neuroblastoma, an aggressive cancer that affects 100 children a year in Britain. They could also help children with a rare brain tumour called medulloblastoma, which is more common in children.

Karen and Kevin Capel, who lost their only son Christopher to a medulloblastoma brain tumour when he was five years old in 2008, are among those calling for the law to be changed. “Children fighting cancer deserve every chance possible,” said Mrs Capel, from Surrey.

The European Medicines Agency, which regulates licensing, admitted there is a lack of development of drugs for childhood cancers.

A spokesman said: “The European Medicines Agency is working actively with its partners across the European Union to address this unmet medical need.”