Losing EU funding for medical research could cost the lives of some vulnerable children, Great Ormond Street Hospital (GOSH) has said.
In a plea to the Government to secure a deal with Europe that protects research and enables the hospital to "retain its brightest minds", GOSH said there is a "serious risk" it could lose EU staff, funding and long-standing EU partnerships.
Since 2010, GOSH, in central London, has received £25 million from the EU - 10% of its funding - for research into new treatments and cures.
With its research partner, the UCL Institute of Child Health (ICH), GOSH is the UK's only academic biomedical research centre specialising in paediatrics.
GOSH said 44 EU-funded research projects are active, with six at clinical trial stage for new treatments. Fourteen research projects are under review, with "uncertainty about current grants, or applications in the future", it said.
It argued that losing EU funding could "critically delay" the development and availability of revolutionary treatments, and because 75% of rare diseases affect children and 30% of children with a rare disease die before their fifth birthday, such research can be the only hope for some families.
GOSH and the ICH also have large numbers of staff from the EU - 24% of research staff, 25% of medical staff and 16% of nurses.
Dr Peter Steer, GOSH's chief executive, said: "We are deeply concerned about what leaving Europe means for our many EU staff and our research programmes. We call on the Government to prioritise the best European deal to protect these essential staff and research programmes, now and in the future.
"For the vulnerable, very ill children we care for, every day counts.
"The UK is known for the strength of its biomedical research. At GOSH our research base enables us to develop transformative treatments and cures and save lives. We do not want to see this eroded by uncertainty about the status of key staff and funding."
GOSH said the freedom of movement of EU patients taking part in UK-EU clinical research is vital to ensure a large patient base for studying rare diseases and some specialist areas such as gene therapy.
This has a knock-on effect for developing new treatments for adults, it said.
It also warned that restricting access to research groups and hospitals across and outside Europe would put the rapid sharing of research and results at risk.
Furthermore, it said drug treatments for children with rare diseases could be affected and the costs of some drugs may rise.
Graeme Warnell's daughter Nina, three, had her life saved during a clinical trial at GOSH for an untested gene therapy.
She had suffered life-threatening infections caused by a rare, aggressive disease of the immune system called severe combined immunodeficiency (SCID).
He said: "It is critical that funding is allocated to the most effective areas of research, to combat rare genetic illnesses that children like Nina have.
"GOSH treats children with the most difficult illnesses known to man. Research must never be compromised or lose priority if we're to continue to save, and value, our children's lives."