Cancer experts have warned that British children may miss out on new cancer drugs post-Brexit because of European clinical trial regulations.
European rules currently allow drug companies to opt out of running clinical trials of new cancer drugs in children.
And after Britain leaves the EU, this loophole could mean youngsters in the UK missing out on European trials and suffering even greater delays in access to the latest cancer treatments, The Institute of Cancer Research, London, (ICR) and The Royal Marsden NHS Foundation Trust said.
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, such as lung cancer - even if the drug's mechanism of action means it could be effective for them.
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.
For example, in the past five years the drugs axitinib and obinutuzumab have received waivers.
But axitinib could potentially be effective in childhood neuroblastoma, hepatoblastoma, medulloblastoma, Wilms tumour and ependymoma. It could also have a role in rhabdomyosarcoma and high-grade glioma.
And obinutuzumab may be relevant for lymphoma.
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.
"By allowing pharmaceutical companies to use waivers to avoid trials in children so they can focus on adult treatments, the regulation is stifling progress and could be stopping children receiving a treatment that could save their lives."
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.
"We've been urging decision-makers to change the regulation for several years now, so that adult cancer drugs are tested in children whenever their mechanism of action suggests they could be effective.
"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.
"It's vital that whatever deal the UK does preserves access to Europe-wide clinical trials for children with cancer, and avoids creating even longer delays in children accessing the latest cancer medicines."
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."