Britain’s opposition Labour Party leader Jeremy Corbyn, congratulates Jim McMahon (not shown), the newly elected member of Parliament for Oldham West and Royton in Oldham, northern England, on Friday.

British opposition leader Jeremy Corbyn passed a key electoral test on Friday when his Labour party won its first parliamentary seat since he took the helm in September.

The win bolsters the embattled leader of Britain’s main opposition amid mounting talk of a leadership challenge, and as the anti-European Union UK Independence Party (UKIP) targets voters in former Labour strongholds.

“It shows the way we’ve driven the Tories back on tax credits, on police cuts, on their whole austerity agenda and narrative,” Corbyn said.

“It shows just how strong, how deep-rooted and how broad our party, the Labour Party, is for the whole of Britain,” he said.

McMahon won the ballot for the Oldham West and Royton constituency with a majority of 10,835, down from a majority of almost 15,000 enjoyed by the last lawmaker to hold the seat, Labour’s Michael Meacher.

His death in October triggered Thursday’s by-election in a part of England that has long identified with left-leaning Labour, but where UKIP is now making inroads with its promises to leave the EU and sharply cut immigration.

UKIP’s candidate came second with 6,487 votes.

Corbyn has had a wobbly start to his tenure as Labour leader, with many party members openly defying his anti-war, left-wing stance, stoking speculation of an eventual coup.

In a parliamentary vote on Wednesday on whether Britain should join US-led air strikes in Syria, some 67 of Labour’s 231 lawmakers voted in favor of bombing.

They included Corbyn’s foreign affairs spokesman Hilary Benn, whose eloquent speech in favor of military action prompted some experts to tout him as Corbyn’s possible replacement.

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