Former Conservative cabinet minister Stephen Dorrell has announced he will stand in the general election as a Liberal Democrat.
He will contest the Buckingham seat, held by John Bercow, who retired as the Commons Speaker on Thursday.
Mr Dorrell said: "I am a liberal conservative, and liberal conservatives should no longer support the Conservative Party."
He stood down as the MP for Charnwood in Leicestershire in May 2015.
Mr Bercow announced in September he would be standing down as an MP.
Mr Dorrell said: "Brexit has become a veneer for English nationalism, and I am against nationalism in all its forms, including its modern Tory form."
In response, a Conservative Party spokesperson said: "We bear Stephen no ill will but a vote for the Liberal Democrats is a vote for more Brexit delay and confusion, two more referendums and Jeremy Corbyn in Downing Street.
"Only a vote for Boris Johnson and the Conservative Party will get Brexit done so we can focus on the police, our NHS and schools and move our country forward."
In the spring, Mr Dorrell joined Change UK, and was the lead candidate on the party's list in the West Midlands region at the European Parliament election, but did not get elected.
He was a cabinet minister in John Major's government, and became an MP in 1979.
He was health secretary during the BSE crisis and a Treasury minister on "Black Wednesday", when the UK's pound crashed out of Europe's ERM.
As a backbencher, he served as chairman of the Health Select Committee between 2010 and 2014.
The deadline for nomination papers for prospective MPs to be returned is by 16:00 GMT on 14 November.
The candidates who have announced they are standing in Buckingham seat are:
- Conservative - Greg Smith
- Labour - to be announced
- The Green Party - Michael Sheppard
- The Brexit Party - Andrew Bell
The general election is taking place on 12 December.