Campaigners have lost a bid at the Court of Appeal to thwart the proposed 4,000-home.

Judges this week backed a previous High Court decision which rejected the claim that the Secretary of State had failed in his duty to call in the application for Mountfield Park development in south Canterbury for deliberation because of environmental concerns.

Because of the protracted legal battle, not a single brick has been laid since planning permission for the scheme was granted in 2016, and the council has yet to issue the formal approval notice.

But the hopes of developer Corinthian Land and the city council that long-overdue building work can now start may still be premature.

Because protestors have already indicated they may now seek permission to take the judicial review to the Supreme Court and are likely to also challenge the planning approval notice when it is released.

The action, brought by environmentalists Emily Shirley and Michael Rundell, has been paid for with a £15,000 fighting fund backed by supporters and sympathetic lawyers. But Corinthians claim the two-year delay has already cost the company millions.

Countering complaints, including those from the developer and the city council, that the legal challenges were denying families much-needed new homes, Mrs Shirley said: "I think it is much more important that people already living here are not breathing in dangerous air pollution.

"There is no point in just keep building more houses if people are being poisoned. It's ridiculous and needs to be addressed.

"I don't think people grasp how dangerous it is to general health - and that is what the science says, not us."

Although the city council granted planning permission for the development two years ago, the formal notice was never issued because of the legal challenge.

During the delays, the developer says it has taken the opportunity to enhance the scheme's 'green credentials', which are expected to be presented to the planning authority in an update to the existing planning application in the near future.

Corinthian says it will contain further measures, including more green travel opportunities and facilities, to mitigate air pollution caused by the predicted thousands of extra vehicle movements, especially in and out of Canterbury.

But even if they are approved by the city council, Mrs Shirley and Mr Rundell say they may still challenge that decision, sparking a new lengthy round of legal wrangling.

The pair also have a separate legal challenge ongoing against the city council's Local Plan, which they say fails to adequately tackle air quality across the district. It has been on hold while the action against the Secretary of State was determined.

Mrs Shirley said: "We are disappointed by the Court of Appeal decision because it is not a rational conclusion and does not stand up to scrutiny.

"Basically, what it means is that neither the Secretary of State or the local authority needs to comply with an EU directive with regards to decision-making about air pollution law, which will be one of the grounds of our appeal to the Supreme Court."

The city council has called the appeal decision a “comprehensive defeat” for campaigners. “The court robustly and unequivocally rejected all of the arguments they put forward,” said spokesman Rob Davies.

“Given this, we would therefore be very surprised if they continued to thwart the delivery of these much needed new homes for local people through an appeal to the Supreme Court.”

Corinthian Land managing director John Trotter says that despite the costly delays, the company is determined to see the development “through to fruition”.

“The judicial review was overwhelmingly dismissed on appeal and we would like to think that we can now proceed with this important development which will provide the much-needed housing that young people and families in the area so desperately need,” he said. “We will monitor air quality throughout the project and will facilitate electric cycle use and electric vehicle charging points.”