But on the eve of the 20th anniversary of the world’s worst nuclear disaster, Ukrainian officials say that they are about to begin an ambitious new project to replace the makeshift shelter.

Work is expected to start next year on a giant steel arc — 108m (355ft) high, 250m wide and 150m long — to enclose the entire block, Ukrainian officials say.

The $1.1 billion (£560 million) reinforced arc will be built to one side and then slid over the block on rails once it is completed in about 2010.

Once in place, experts will start to dismantle the sarcophagus and remove the estimated 200 tonnes of radioactive material from inside, a process that could take as long as 50 years.

If successful, the plan should safeguard the site for another 100 years, according to officials. But they are already giving warning that it promises to be one of the most complicated construction projects undertaken.

“This hasn’t been done by anyone before,” said Jean-Louis Le Mao, bid manager for Vinci/Bouygues, a French consortium competing for the contract. “It is a real prototype.”

The other company bidding for the contract, CH2M Hill, based in Colorado, did not provide any details of its proposal, but Mr Le Mao said that his consortium’s plan would need two and half times the amount of steel used to build the Eiffel Tower and more than 1,000 workers.

The 800 workers on site would have to wear protective clothing and respiratory masks to protect themselves from the radioactive dust within the 30km (19-mile) radius of the contamination zone.

“The fact is we’d have to build this very carefully,” he said.

Yuliya Marusych, a spokeswoman for Chernobyl, issued a warning that the sarco- phagus — made out of 400,000 cubic metres (14 million cu ft) of concrete and 7,000 tonnes of metal — is slowly collapsing.

Rainwater has leaked in and one of the four towers inside is collapsing. And, as only 25 per cent of the reactor is accessible, no one knows exactly how much radioactive fuel remains inside, she said.

The winning bid for the project was supposed to have been selected in February, but the process has been delayed by political gridlock in Ukraine after the parliamentary elections last month.

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