Britain is coming under increasing pressure to provide Ukraine with an extra €50m (£43m) to construct a new contamination shield over the top of the stricken Chernobyl nuclear plant before the old one collapses.

Officials from the European commission said governments around the world were being urged to find €750m to help build a more sophisticated roof over the burnt-out reactor and storage for 200 tonnes of highly radioactive fuel.

Jean-Paul Joulia, from the commission's nuclear safety unit, admitted the cost of just this aspect of the Chernobyl clean-up was running at €1.5bn – double the original estimate – partly due to "some delays" to some projects.

But he said he was confident that foreign governments would stump up the money needed for the shield, even in today's financially difficult climate. "I am optimistic the international community is committed to this. It is important for a number of reasons," he said.

The disaster at Chernobyl, on 26 April 1986, is recognised as the world's worst nuclear accident. One of the power station's reactors exploded and the subsequent fire spewed a radioactive cloud across Europe.

The accident claimed the lives directly of at least 50 people, mainly fire crews and nuclear workers who tried to fight the fire on the fateful night.

Radioactive fallout is believed to have caused many other deaths from thyroid cancer and related illnesses, with an eventual death toll estimated at anywhere between 4,000 and 200,000.

The disaster also triggered the relocation of tens of thousands of local people, some of whom have never been allowed back to the contaminated towns.

The accident and its aftermath have resonance currently as new nuclear plants are being planned in Britain and other parts of Europe.

Nuclear power was heavily discredited for some time after Chernobyl because the accident pointed to the enormous dangers associated with it.

Ukraine and foreign contributors have already spent more than €750m trying to make safe the nuclear plant. The UK has so far provided €53.1m to help build the new shield and is under pressure to stump up a further €50m, although Ukrainian sources said no money had yet been promised.

With the 25th anniversary approaching, the European commission is encouraging governments to make public new cash commitments in advance of a "pledging" conference scheduled for one week before the April anniversary.

The European Bank for Reconstruction and Development, which has been overseeing the spending of the money at Chernobyl, admitted squeezing out more money at this time was a "big challenge".

Vince Novak, a director of the nuclear safety department of the EBRD in London, said the design life of the temporary shield thrown up by the-then Soviet Union in the immediate aftermath of the accident in 1986 "runs out" in 2016.

A new moveable barrier would be one of the world's biggest engineering feats due to its size and innovative features. The work will be done by a consortium involving Bechtel, the US firm which has worked on the London Underground and the Channel tunnel.

The current shield has been propped up by a series of new steel columns but experts working on the project accept that the structure is barely adequate and needs replacing as quickly as possible.

The barrier is sitting over the top of what remains of a deeply radioactive reactor core, but also covers the vast bulk of the fuel from the plant which has seeped into the concrete floor.

A new permanent shield would prevent rain from getting into the old plant but also allow more work to be carried out on dismantling the remains of the equipment and the fuel without creating more escapes of hazardous dust and particles.

Fallout over UK

Nuclear workers in Sweden were the first to detect the radioactive material that was thrown into the atmosphere from Chernobyl. The fallout from the accident in the early hours of 26 April 1986 crossed over Europe, and deposited the radioactive isotope caesium-137 in mainly upland areas of Wales, Scotland and England.

The disaster, which released at least 100 times more radiation than the atom bombs dropped on Nagasaki and Hiroshima, forced the government to put 9,700 farms and 4.2 million sheep under restriction across the UK.

Restrictions were lifted in Northern Ireland in 2000. The Department for Environment, Food and Rural Affairs confirmed yesterday that the last time the government updated the public on the restrictions was in 2009 when the then health minister Dawn Primarolo revealed 369 farms and 190,000 sheep were still affected.

Of those farms, 355 are in north Wales, with nine in Cumbria and seven in Scotland. Farmers in these areas must have their livestock scanned before they are able to move them.