Campaigners have launched legal action against Merton Council claiming they mislead the Mayor of London over controversial plans to expand a primary school.

Earlier this month the Mayor of London said he was "content" for the council to move forward with plans to expand Dundonald Primary School in Fairlawn Avenue by building on part of Dundonald Rec - plans approved at a planning committee meeting on January 17.

The expansion will create 210 extra places with a new two storey extension comprising of three new classrooms, a school hall, new studio, kitchen and library planned.

A multi use games area, tennis courts, playground and outdoor gym will also be created but the current bowling green will be lost. 

But campaigners had hoped Mr Johnson would overturn the decision.

Lorraine Maries, chairman of Protect Dundonald Rec, said they were "disappointed but not surprised" at his approval but claimed he had been misled. 

She said: "The council has acted in a completely undemocratic way.

"This application has produced the largest number of objections that the council has ever seen.

"The voices of local residents, Friends of Dundonald Park and the users of the recreation ground have been totally disregarded."

"This matter is now in the hands of our solicitors, Richard Buxton, so we cannot comment further, but we can assure local people and Rec users that it is highly unlikely this building will go ahead."

The expansion could still be halted if a lands tribunal, due to take place later this year, decides the plans breach a historic covenant restricting building work on the site. 

Councillor Martin Whelton, cabinet member for education, said: "We await that and we have put forward a case but we hope that work will get under way later this year on what is a much needed extension to a very good school.

"As a council we welcome the decision of the GLA - it’s a decision for common sense.

"We have a huge demand for new places and it will also enhance sporting facilities in the park."

The Mayor’s approval comes as a National Audit Office report said a quarter of a million extra school places would be needed in England by autumn 2014.

Merton Council has created 630 additional primary school places over the last four years, but warned that a lack of land and government funding would limit local authorities’ ability to continue to meet demand for primary school places.

Liberal Democrat spokesperson Diana Coman said: "The argument against the plans to build on the Rec is about the erosion of an asset.

"We have a problem of lack of school places, but using other assets - like public open space - to correct the problem created by previous council administrations does not seem to me to be an answer.

"I had written to Boris asking him to consider these issues in making his decision.

"Sadly he’s just ignored these arguments."