Known to its designers as “the Ark”, the arch-shaped tubular structure, 360ft high and 900ft across, will make safe the site of the world’s worst nuclear accident when it is finally given the go-ahead.

Scientists and international aid donors who will meet in Kiev, the Ukrainian capital, this week on the 20th anniversary of the accident were hoping to announce approval for work to begin on the Ark.

But the project has become embroiled in wrangling between the donors, the French-led engineering consortium and the Ukrainian authorities over the tendering procedure.

The massive structure, officially called the New Safe Confinement, is designed to cover the hastily constructed “sarcophagus” that encases the highly radioactive remains of Number Four reactor.

The sarcophagus was built within months of the disaster, with helicopters lifting slabs of concrete into place to cover the devastated reactor building.

An estimated 200 tons of radioactive matter lies within the temporary structure but the sarcophagus and everything within it are contaminated.

The European Union and other international donors have spent tens of millions of pounds on stabilising the structure, which many had feared would collapse, releasing its deadly contents in another calamity.

The new shield has been designed to contain the radioactive remains for the next 100 years. The Ark is intended not only to enclose the site but to permit work by remote- controlled devices or specially trained teams to dismantle and store the lethal material safely.

Large prefabricated portions of the arches will be brought to Chernobyl and assembled in two halves at a distance from the sarcophagus to minimise workers’ exposure to radiation. The final operation to lock the two parts together will be performed within 24 hours by sliding them into place on a specially constructed railway line.

To enable the ruined reactor to be dismantled, the Ark has been designed to carry four bridge cranes which will be suspended from the arches. Each crane will be capable of lifting 100 tons. Railway carriages shielded against the radiation will transport workers deep into the bowels of the new structure.

Ukraine’s political instability since the Orange revolution 15 months ago — three ministers have been responsible for the scheme in that time — has added to the air of uncertainty surrounding the Ark project.

The funds for the programme, to total £600m, are being administered by the London-based European Bank for Reconstruction and Development. Vince Novak, the director of the bank’s nuclear safety department, said: “It is disappointing we won’t be able to declare on the 20th anniversary that work on the new shelter is to commence.”

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