The Tate has also gained planning permission from Southwark council for its plans to transform the striking building on the south bank of the river Thames in London, Tate director Nicholas Serota said.
English Partnerships' money goes to Magnox plc, the publicly owned section of the nuclear industry which owns the site and is decommissioning it.
Mr Serota said: "This investment will help turn a derelict site at the heart of the capital into a major cultural, social and economic asset for Southwark and the nation."
The project was also recently awarded a pounds 50m grant by the National Lottery- backed Millennium Commission, and it is hoped to open it in 2000.
The Tate Gallery must find the rest of the estimated total of pounds 106m cost from other sources. Mr Serota said remaining money had to be found by February next year.
But he added: "I can assure you that we have a large number of commitments and we are making really excellent progress in getting that money and are well on our way to reaching that pounds 56m."
Yesterday's pounds 12m regeneration grant was the first part of that, he said. English Partnerships chief executive, David Taylor, said the grant was unconditional.
"This investment will help turn this redundant and derelict building into a major national landmark," he said.
"It will create up to 1,000 new jobs for people in Southwark and will promote the broader regeneration of the area."
It was the first time, perhaps in Europe, that a former power station had been reclaimed for new use, he said.
The Department of Environment had spent millions decommissioning old power stations, as had the Department of Energy and CEGB after it, he said.
Magnox Electric chairman Mark Baker said: "Not only will Bankside get continued life as a valuable public asset, the taxpayer has benefited too."Reuse content