The boys will have a normal life expectancy if they can find donors

Joshua, 12, Nathan, 10, Daniel, eight, and Luke Hartley, four, suffer from X-linked lymphoproliferative disease (XLP), which attacks the immune system.

Only 100 people in the world have so far been diagnosed with XLP. Sufferers do not usually live into their teens.

But a full bone marrow transplant would give the boys, from Romsey in Hampshire, normal life expectancy.

A worldwide search by the Anthony Nolan Trust, which maintains a register of bone marrow donors, has, so far, proved unsuccessful.

Allison is waiting to find out if she can give bone marrow to Joshua

But their parents, Dave and Allison Hartley, fear time may run out before a donor is found.

Dave Hartley, said: "It was described to us that XLP is like a time bomb, and that time bomb is ticking and that means that things can go wrong - potentially very rapidly."

Mr and Mrs Hartley learned just before Christmas that their sons had XLP, which was discovered in 1999 and affects only boys.

Joshua already suffers from acute anaemia, while Nathan was diagnosed with lymphoma when he was three; both conditions are also caused by immune system problems.

'Worst nightmare'

Mr Hartley said: "I can't put it any other way, it is the worst nightmare for any parent. We were clearly gobsmacked when we were told the news."

The family is organising a recruitment drive at a bookshop in Romsey, where people can give a small sample of blood to see if they could be a potential bone marrow donor for any patient in need of a transplant.

Mrs Hartley learned last week that she was a match for her eldest son Joshua and doctors are waiting to see if she will be able to donate her bone marrow.

The children have all been told about their condition and know the importance of finding donors.

Mrs Hartley said: "We are a close family and have talked through things with them.

"Obviously they have their own concerns which we have just been able to talk about and help them work through."

Anyone who is healthy and aged between 18 and 40 can join the Anthony Nolan Trust register and see if they are a match for the boys.