Campaigners have vowed to push ahead with their legal challenge against the council’s approval of a new Catholic school, despite the huge costs of a judicial review.

Richmond Inclusive Schools Campaign (Risc) and the British Humanist Association (BHA) have hired solicitors Leigh Day and Co, which wrote to the authority on March 29 to warn of the planned action.

Sarah Willis, a senior lawyer at Richmond Council, replied on April 5 denying it had taken any unlawful decision because it had not yet offered the site, in Clifden Road, Twickenham, to the Diocese of Westminster.

The authority then announced on May 24 that it had approved a new Catholic primary and secondary school.

Jeremy Rodell, of Risc, said the group has asked the council to co-operate to complete the legal proceedings as soon as possible, because parents faced an uncertain delay before enrolling their children for the new academic year starting September, 2013.

He said: “To reduce the period of uncertainty we have asked the council - and our lawyers will ask the court - to do whatever they can to speed up the proceedings, though we have to recognise that we’re heading into the holiday period, so the timing may not be ideal.

“We also asked the council to agree to a reasonable limit on the legal costs that either side can claim from the other if it wins.

“We had expected it to agree, so both sides would have an incentive to keep costs to a minimum and they could limit the amount of public money at risk. But so far it has refused, and has instead tried to intimidate us on costs, presumably because it has plenty of taxpayer funds available.”

Lord True, leader of Richmond Council, said it had made a democratic decision as part of an open and accountable process and he was disappointed by BHA and Risc’s decision.

He said: “The council will defend this local decision against any legal action confidently and vigorously, seeking costs against any challenger.

“I ask those contemplating such action to ask the British humanists to respect localism and local choice and to reflect that every penny spent on lawyers would be money lost to spend on the education of all young people in the borough.

“That, we all agree is the paramount objective. Every month spent in the courts would also be a month lost in preparing new schools for hundreds of local children.”

Risc and BHA argued the Government’s new Education Act required any council needing a new school to first consider proposals for an academy or free school, which can enrol a maximum of 50 per cent of pupils based on religion.