The amount of the deal was not disclosed, but the Spanish business paper elEconomista said it could be about 20 billion euros (28.5 billion dollars).
Iberdrola Renovables made the bid through its affiliate ScottishPower Renewables.
The wind farm, which would have an energy capacity of 7,200 megawatts, would be built in the North Sea off the eastern county of East Anglia, Iberdrola Renovables said in a statement.
Iberdrola Renovables and Vattenfall have formed a 50-50 joint venture company to build the project.
Source: AFP European Edition