Mark Reckless has launched a campaign against the scheme at Lodge Hill, despite supporting the proposals when he was formerly a Conservative MP.
Reckless announced last month that he had had joined UKIP. A by-election will be held in his Rochester and Strood constituency next month.
The Lodge Hill development, which was granted consent by Medway Council last month, proposes to deliver 5,000 homes, alongside business and retail space, schools, healthcare and community facilities, hotels and associated infrastructure.
Reckless is now petitioning the government to call in the decision and have the proposals reconsidered by a planning inspector.
Writing on his website last week he said: "This is justified because the country’s whole system for protecting the environment will be undermined if building is allowed in this Site of Special Scientific Interest that is home to the UK’s largest nightingale colony."
However, while a Conservative MP Reckless was in favour of the scheme, saying in March last year that it would deliver "5,000 much needed homes".
Speaking in Parliament he said: "The comparison to be drawn is between those 84 nightingales and homes for 12,000 people and jobs for a further 5,000 people. We are told by the Prime Minister that we are in a global race, but it is not clear that that message has yet filtered through to bodies such as Natural England."
Reckless’ apparent turnaround drew criticism from former planning minister Nick Boles.
Boles said on Twitter: "Breathtaking hypocrisy by @MarkReckless: he lobbied me repeatedly in favour of development at Lodge Hill. Rochester & Strood deserve better."
Efforts were made to contact Reckless but he had yet to respond at time of going to press.