MADRID (Reuters) - Spanish utility Iberdrola (>> Iberdrola SA) has agreed to sell its 50 percent stake in British nuclear consortium NuGen to Japan's Toshiba (>> Toshiba Corp) in a deal that could help rejuvenate Britain's nuclear construction plans.

NuGen, a joint venture between Iberdrola and French utility GDF Suez (>> GDF SUEZ), owns a site at Sellafield on the northwest coast of England where its plan to build 3.6 gigawatts of nuclear capacity has stalled amid government delays.

Britain has a nuclear new build programme to replace aging and polluting power plants by the middle of the next decade and says it is keen to encourage investment.

But regulatory changes after Japan's Fukushima accident and squabbles about how much public money should be handed to companies for new plants have slowed progress.

Iberdrola said on Monday it had sold its NuGen stake to Toshiba, which owns nuclear plant builder Westinghouse, for 85 million pounds ($139 million) or 102 million euros.

Iberdrola has a large presence in Britain as owner of utility Scottish Power but has been selling assets to reduce debt.

The agreement, which will require regulatory approval, will give Toshiba a foothold in Britain, where Hitachi Ltd (>> Hitachi, Ltd.), Japan's other nuclear technology supplier, acquired the Horizon nuclear project last year and is planning to build two to three nuclear power plants.

Britain in October approved a deal with French utility EDF Energy (>> EDF) to build the 16 billion pound Hinkley Point nuclear power plant in Somerset.

(Reporting by Tracy Rucinski; editing by Jason Neely)