Patients are left in isolation units while waiting for donor marrow
The ban on air travel in much of Europe is putting the lives of people waiting for bone marrow transplants at risk.
The disruption caused by the Icelandic volcano ash cloud means patients have been left waiting for treatment.
In the UK, 16 bone marrow treatments scheduled over recent days have not taken place, the BBC has learned.
The Anthony Nolan Trust, which arranges the worldwide transport of bone marrow, says couriers have had to use trains and ferries to enter the UK.
The bone marrow is used to treat leukaemia and lymphoma.
A spokesman for the trust said bone marrow had to be moved within 72 hours before the cells died.
"The bone marrow which goes from a donor to a patient can't be stored like pints of blood.
"The donor only donates when they are sure there is a person who needs treatment somewhere in the world."
Some of the marrow due to come into the UK was coming from Europe, while other donor material is coming from the US.
The spokesman said patients undergo treatment which effectively destroys their own immune system, which means they are left in isolation units in a "very fragile" state until they can receive new bone marrow.
More treatments are likely to be postponed if the flight restrictions continue. The ban on flights in UK airspace has been extended until at least 0100 BST on Monday, with the volcanic ash likely to stay over the UK for many days.
The material is transported by volunteer couriers who have to carry the marrow in plastic containers as hand luggage and cannot let them out of their sight.
The trust has urged people to continue signing up to their donor register, as bone marrow matches will still need to be made months and years into the future.