The guard towers of Auschwitz are splintering, the barracks are waterlogged: the concentration camp where one million Jews were slaughtered is decaying so fast that conservationists have called on Britain to help to save it.

The theft last month of its distinctive, sinister sign, Arbeit macht frei (work sets you free) has underlined the vulnerability of the Nazi death camp, stretching over 20 hectares (50 acres) of southern Poland.

“Nobody could have imagined such a horrific act of vandalism,” Jacek Kastelaniec, director-general of the Auschwitz-Birkenau Foundation, said. “Now try to imagine the public outcry if one of the barracks started to fall down, impossible to restore.”

Auschwitz was built on boggy ground between two rivers; as a result the high groundwater and bad drainage has rotted the foundations. Walls are blistering and starting to lean, roof frames are buckling, plasterwork and wall-paintings are flaking.

Mr Kastelaniec will go to the Cabinet Office tomorrow to press the Government on Gordon Brown’s promise to contribute to a €120million (£110million) endowment fund that will guarantee the preservation of one of the main sites of the Holocaust.

Mr Brown visited the camp last April, and, plainly upset by what he had seen, declared: “We will join with other countries in supporting the maintenance and retention of the memorial at Auschwitz.” No figure has been suggested publicly for Britain’s possible contribution, but Polish sources say that the conservationists are hoping for about €10million.

Angela Merkel, the German Chancellor, has said that her country would put up half of the costs, but the managers of the Auschwitz museum need other commitments. Mr Kastelaniec will also visit France, Belgium and the United States. The Polish Prime Minister, Donald Tusk, has sent an appeal to 40 heads of government.

“The conservationists say we need to start work in the next two years if we are to avert irreparable decay,” Mr Kastelaniec told The Times, “and that will only be possible if the money is paid into the fund now.”

The decay of the camp is politically sensitive. The current trial in Munich of the alleged Sobibor camp guard John Demjanjuk is being seen by the public as the last for Nazi war crimes — the 89-year-old defendant is wheeled into court on a hospital bed.

Holocaust survivors are dwindling. “In ten years there will be no witnesses,” Mr Kastelaniec said, “and it will be easier for the crazy people who say nothing happened in the camps.” Only the buildings will remain.

Auschwitz cannot simply have a makeover because that would undermine its claims to authenticity, and open the way for those on the far Right who try to deny or trivialise the Holocaust. The strategic point of the restoration is to use its almost over-powering sense of menace as a clinching counter-argument against anti-Semitism and racism.

The portfolio to be presented to the British Government underlines the vast scale of the camp. The priority is being set on 45 brick barracks. The managers estimate that it will cost up to €890,000 to restore a single barracks building. On top of that come 22 wooden barrack rooms — where inmates were crowded into bunks up to the ceiling. Each will cost €310,000.

Then there are the remains of 210 barrack buildings. Some sheds have collapsed, but there are concrete outlines where floors and chimneys stood. Without some strengthening, these foundation markings will disappear. Cost: €78,000 per barrack room. The 27 wooden guard towers need to be reinforced at an annual cost, for the next 14 years, of €62,000.

Work is under way on conserving the ruins of the gas chambers and crematoriums, but the managers want to extend this to include a provisional gas chamber-bunker, two other crematoriums and the unloading ramp.

Property taken from prisoners before they were gassed, now exhibited in small piles in the museum, is also showing signs of age: 460 artifical limbs, 40kg (90lb) of discarded spectacles, 260 prayer garments and 3,800 suitcases that belonged to people who ended their journey in Auschwitz.

The sluggish response worldwide to the restoration had been down, in part, to the feeling that the main burden should be on Germany. Mr Kastelaniec said: “The breakthrough came when we convinced not only Germany but also other contributors that this was not a project about guilt, but about the future.”

Remembering the victims

Rwanda Kigali Memorial contains photographs and remains of victims of the 1994 genocide

Cambodia The Khmer Rouge killing fields have been opened to tourists, with the remains of 8,985 victims on display

South Africa Robben Island, where Nelson Mandela was imprisoned under apartheid, has been a heritage site since 1997

Spain El Valle de los Caidos, a vast cross and tomb near Madrid, was built by General Franco to honour fascist casualties in the Civil War