British Prime Minister David Cameron faces a potentially awkward test of voter support following the resignation of lawmaker Patrick Mercer, a former member of his Conservative Party.
Mercer's resignation on Tuesday, after reports he was to be suspended for six months for breaking lobbying rules, triggers an election that would allow the anti-European Union UK Independence Party (UKIP) to test its growing popularity in a region where the Conservative Party is traditionally strong.
The result could reveal how much support the Conservatives, who in 2010 had a healthy 16,000 vote majority in Mercer's Newark constituency in the English Midlands, have lost to UKIP.
The vote will not take place until after European Parliament elections next month, in which UKIP is expected to push Cameron's Conservatives into third place behind itself and the opposition Labour party.
Minority parties tend to do better in European than in domestic elections as the voting system favours them and voters are more likely to cast their ballots in protest or on single issues.
So a big loss of Conservative votes in Newark to UKIP, which has nine seats in the European Parliament but no elected members in Britain's parliament, could hint at a serious split in the centre-right vote in next year's national election.
UKIP leader Nigel Farage - who has enjoyed a huge rise in media coverage ahead of the EU elections - had said he was "seriously considering" standing for the seat, a party spokeswoman said, but he eventually decided not to run.
The parliamentary standards watchdog is due to publish a report today into allegations uncovered by a television documentary last year that Mercer, a member of parliament since 2001, offered to use his influence for personal gain.
Local media reported on Tuesday that the committee would recommend suspending him from parliament for six months.
"What has happened, has happened and I am ashamed of it. Therefore I am going to do what I can to put it right," Mercer told reporters outside parliament. "I am going to resign my seat."
Mercer stepped down from the Conservative party in May last year when the allegations came to light and at the time also said he would not seek re-election next year.