The death of Michael Meacher means that the first electoral battle of the Jeremy Corbyn era will come sooner than we thought: a by-election in the safe Labour seat of Oldham West and Royton. How worried should the party be?

On paper, the seat is safe as houses - Meacher held the seat with a majority of 14,738. Ukip are a distant second with 8,892 votes, and with the Conservatives just a handful of votes behind them in third place. But Labour strategists are all too aware of a by-election just over a year ago in the neighbouring seat of Heywood and Middleton, when Ukip came from nowhere to within milimetres of taking the seat from Labour. The seat is almost identical to Heywood & Middleton, although, to Labour's good fortune, it has a significantly larger south Asian population.

That ought to make it a trickier prospect for Ukip, but party staffers fear that the party's "Ukip problem" has got worse in the year that has passed. In a way, the good news is, no-one in the party now argues that Ukip is a Conservative problem. Until the election, senior aides would still argue that Ukip "helped Labour". Now, after losses to Ukip contributed to defeats in Southampton Itchen, Telford, Plymouth Moor View, Morley & Outwood, and Gower - which Labour had held since the party was founded - no-one argues that Ukip is anything other than a threat to Labour.

But the evidence from council by-elections and from the polls - and, I'm told, from the party's own canvassing - is that Labour's Ukip problem has got worse since Corbyn became leader. Under Ed Miliband, the party was shedding votes to Ukip and the Greens - and was uncertain about what to do to fix the problem. Under Corbyn, Labour's Green problem has almost ceased to exist. The Greens are believed to have shed at least 2,000 members since Corbyn became leader and their vote has gone down in every council election they have fought since Corbyn took office. But gains among the Greens have been offset by increasing losses to Ukip.

The good news for Labour is that Ukip are preoccupied and divided by the coming referendum on Britain's membership of the European Union, while Labour still have a Rolls-Royce operation as far as by-elections are concerned. The party's ruling NEC will select a shortlist, and Labour is likely to continue their practice of the last parliament of holding their by-elections quickly, to prevent rival parties getting their foot in the door. Labour's ground team is confident of holding the seat if the by-election is held quickly - but a longer campaign, and a well-organised effort from Ukip, could see the party given a bloody nose.