The construction of a new sarcophagus above the defunct Chernobyl Atomic Power Station will begin on April 26, the 26th anniversary of the world's worst nuclear disaster, Ukraine said Tuesday.

Preliminary work on the 20,000-ton structure designed to contain threats from radioactivity began earlier this year.

Ukraine's President Viktor Yanukovych said in a statement that full construction "will begin on April 26".

The disaster contaminated large parts of Europe but especially the then-Soviet republics of Ukraine, Belarus and Russia.

"The needed funds are available and the work is conforming to the calendar," said Anton Ussov, spokesman for the European Bank for Reconstruction and Development (EBRD), the project's largest funder.

He added that the new enclosure should "be completed and installed above the damaged reactor in mid-2015 as expected."

After the disaster, the Soviet authorities put up a supposedly temporary concrete shelter to protect the destroyed reactor but there were persistent doubts as to its durabilty.

The new structure is estimated to cost 1.5 billion euros ($2 billion).

The international community had struggled to find funding for the project, but the needed resources were secured following a conference in Kiev last year that marked the 25th anniversary of the disaster.

Donors have agreed to contribute 550 million euros to the project, with the balance coming from the EBRD.