CORBY — The ruling Conservatives of Prime Minister David Cameron lost the key marginal seat of Corby to the opposition Labor party in a parliamentary byelection, according to official results Friday. Corby, in central England, has voted for the winning party in every general election since 1983, making the one-off vote, triggered by the resignation of Conservative lawmaker and novelist Louise Mensch, an important political marker. The Labor candidate polled 17,267 votes to the Conservatives' 9,476, overturning Mensch's slim 2010 majority of 1,951. The UKIP party, which wants to pull Britain out of the European Union, also polled strongly with 5,108 votes. Prime Minister Cameron is already under pressure from within his party as several senior members had raised a banner of revolt against him over Britain's ties with the European Union.— AP