Basingstoke MP Andrew Hunter has joined the Democratic Unionists under what is being described as a "unique arrangement".
Mr Hunter has said he will stand down as MP at the next election.
Mr Hunter, who has been sitting as an independent Conservative since 2002, becomes the party's first English MP.
A DUP spokesman said the MP was merely formalising a link, having been a close friend and supporter of the party for some time.
Mr Hunter has already said he will be standing down at the next election.
He narrowly missed out on a seat in last year's elections for the Northern Ireland Assembly.
Mr Hunter's defection to the DUP brings the party's number of Westminster MPs to seven.
The Labour candidate in Basingstoke for the next general election, Paul Harvey, has called for Mr Hunter to seek a new mandate in a by-election.