Harcourt, which bought the Tory Party headquarters in 2007, has sold it on to European legislators.
The Irish developer bought the property last year from the Conservatives for £30.5 million. The building at 32 Smith square will now be used as a joint headquarters for the European Parliament and the European Commission.
Property Week has reported the new owners will now appoint architects to equip the building with eco-friendly technology.
Gerard Onesta MEP, vice president of the European Parliament, said: "Our ambition now is to refurbish the building to the highest standards of environmental excellence, fit for the 21st century, to serve the citizens of the UK."
The publication has claimed Harcourt’s initial intentions were to convert the building into flats.
Mr Onesta was pleased with the deal, which saw ownership pass to the European politicians for £10 million less than it cost Harcourt.
He told Property Week: "This deal will provide a suitable and convenient Europe House in the UK."