The byelection caused by the death of veteran MP Michael Meacher will be held on 3 December, providing the first electoral test of Jeremy Corbyn’s leadership, party sources have confirmed. Labour is expected to move a writ in the House of Commons on Monday, formally triggering the byelection process in Meacher’s Oldham West and Royton constituency.

At May’s general election, the former environment minister held the seat with a majority of 14,738 over Ukip. The forthcoming byelection is now seen as an important moment for Corbyn, who claimed during his leadership campaign to be able to win back Labour votes from the anti-EU party.

One senior Labour MP said that he was surprised at the haste with which the byelection was being called. He said this suggested that the leadership wanted to move fast and prevent Ukip having an opportunity to launch a long campaign that could seriously challenge Labour.

There has been intense speculation over whether Corbyn and his supporters would try to parachute a leftwinger into the seat. Figures close to the selection last night expressed surprise that several local councillors had not been longlisted by a three-person committee that included Labour MP Keith Vaz. But the strong favourite for the selection is Jim McMahon, the leader of Oldham council, who is seen as a moderate. The 35-year old McMahon had previously been expected to seek the party’s nomination for the new role of Greater Manchester mayor.

Though Ukip finished a distant second to Labour in May, Labour insiders are worried that support for Nigel Farage’s party could swell significantly in this contest. In the 2014 by-election in the neighbouring constituency of Heywood and Middleton, Ukip came very close to taking the seat from Labour, receiving only 617 fewer votes.

Ian Warren, a political analyst who advised Labour on how to combat Ukip at the general election, told The Observer that the two seats are demographically “identical” except for a large minority ethnic population in Oldham West and Royton. Because of this and “residual loyalty towards Michael Meacher” he perceives Ukip to be “a lower threat than they were in Heywood and Middleton”.

Warren believes that a quick campaign will make it more difficult for Ukip to put up a strong fight. He said: “Ukip don’t have the people on the ground. They’d love it to get into the new year so they can draw people in from neighbouring seats to help out. They can build it slowly and then develop a narrative around Jeremy Corbyn.”