The opposition Labor Party has snatched a Welsh parliamentary stronghold from the governing Conservatives in the third electoral upset for Prime Minister John Major since he took office in November.

"This has been a watershed by-election," Labor campaign manager John Cunningham declared after Thursday's balloting. "It demonstrates conclusively Labor is on course to win a general election. We are winning in Tory heartlands."Labor took the relatively prosperous rural district of Monmouth, controlled by the Conservatives for 25 years, with 17,732 votes for its candidate Huw Edwards to 15,327 for the Conservatives' Roger Evans, results showed Friday.