The UK Independence Party has chosen MEP Roger Helmer to fight the Newark parliamentary by-election next month.
Mr Helmer, who defected from the Conservatives in 2012, is currently UKIP's energy spokesman.
Party leader Nigel Farage decided against standing in the contest, triggered by former MP Patrick Mercer's resignation over a lobbying scandal.
The Conservatives have selected Robert Jenrick to fight the 5 June contest. Michael Payne will represent Labour.
The Lib Dems are to choose their candidate in the next few days.
The MEP said he regretted that the announcement of his candidacy had been overshadowed by criticism of his views on rape and gay marriage.
On his blog in 2011, before he left the Conservatives for UKIP, he had detailed a hypothetical "date rape" scenario before concluding that "the victim surely shares a part of the responsibility, if only for establishing reasonable expectations in her boyfriend's mind".
Mr Helmer has also argued on a blogpost, published on the day he joined UKIP, that same-sex marriage is a slippery slope towards legitimising incest.
He told the Daily Express: "I wish I hadn't written it as its become the only story. But as I've said again and again, these aren't the top issues for the people over Europe.
"I have been campaigning for the European elections for a couple of weeks and nobody has raised these posts with me."