British Children with cancer could be at risk of missing out on drug trials after the UK leaves the European Union because of European clinical trial regulations, say experts.
European rules currently allow drug companies to opt out of running clinical trials of new cancer drugs in children and instead carry them out on adults.
The Institute of Cancer Research, London, (ICR) and The Royal Marsden NHS Foundation Trust say this loophole could mean youngsters in the UK suffer even greater delays in getting access to the latest cancer treatments.
The organisations have called on the European Commission to close the loophole in the EU Paediatric Regulations.
The rules, introduced in 2007, allow pharmaceutical companies to apply for a waiver from having to trial a cancer drug in children if it targets a cancer that does not affect children.
A new analysis by The ICR shows that, over the last five years, pharmaceutical companies were granted waivers from having to trial cancer drugs in children for 33 of 53 cancer treatments.
Almost two-thirds of these treatments were ultimately approved.
But some drugs which are targeted at adult cancers may be effective to treat childhood cancers, they added.
The organisations said that nowhere near enough cancer medicines are being trialled in children or licensed for paediatric use.
They are calling on the European Commission to revise the waiver system so that adult cancer drugs have to be tested in children whenever there is evidence that they could be effective.
"This regulation was introduced to improve children's access to the newest medicines but it's not fit for purpose," said Professor Louis Chesler, team leader in paediatric solid tumour therapeutics at The ICR and consultant in paediatric oncology at The Royal Marsden NHS Foundation Trust.
ICR chief executive Professor Paul Workman added: "Children with cancer are currently missing out on the kind of innovative cancer treatments that are becoming increasingly common in adults because of outdated European rules that have failed to keep up with advances in science.
"This is a real chance for reform to prevent the current out-of-date approach from being cemented for a decade.
"It could also be the last chance to make meaningful changes that apply across Europe, including the UK, before we leave the EU.
A Department of Health spokesman said: "Ensuring young patients have timely access to safe, effective medicines is, and always will be a priority.
"In fact Brexit brings opportunities in this area to secure even faster access to the latest innovations.
"Involvement in clinical trials is a key part of this, and our commitment to significantly increase young people's access to clinical trials, as recommended in the Cancer Taskforce, will not be weakened by leaving the EU."