Labour has failed to increase its vote share in two dozen council byelections across the country stretching back more than two months, a polling analyst has said, indicating the potentially difficult task ahead for the party at the next general election.

The last time Labour’s vote share rose was on 8 August, and since then the party has failed to match this in 24 other seats, including key marginals such as Worcester, Ipswich, Canterbury and Crawley, according to the analysis by Robert Hayward.

Hayward, who is a Conservative peer but also a well-known elections analyst, said that while local factors often skewed individual council byelections, the number of seats and the range of locations – in Scotland and Wales as well as England – seemed to point to a pattern.

“It is an indication of the scale of the problem that they are facing,” Hayward said. “We know about the Tories. All the attention has been on the Tories, but I think these figures highlight the sheer volatility of the voters in relation to Labour support.”

In recent national polling, Labour has tended to be behind the Conservatives by some way, on about 24% or slightly more. The party was also polling weakly before the 2017 general election, but recovered to deny Theresa May a majority.

Hayward’s analysis showed there had been 28 council byelections since 8 August, when Labour last increased its vote share, in a Northamptonshire district council seat.

Of these 28, Labour did not contest two, and in two more had not contested the previous time. But of the other 24, the party’s vote share dropped compared with the previous time the councillor who created the vacancy was elected, the standard metric Hayward uses.

Areas where the Labour vote dropped included seats the party won in 2017 and would hope to hold, such as Canterbury, Ipswich and Cardiff North, and Labour target seats such as Crawley and Worcester.

In one byelection in Cambridge, the swing to the Liberal Democrats was so big that if replicated in the parliamentary constituency, the Lib Dems would take it from Labour, despite the current 12,000-plus Labour majority.

During this run, Labour gained a council seat from the Conservatives, in Luton, but only because the Tory vote dropped even more than that of Labour.

Overall, in the same period, the Conservatives contested 28 seats, with the party’s vote share increasing in 14 and dropping in 14.

Hayward said: “It’s come as a surprise to me, the extent of the challenge, that there should have been all these byelections in the last two months, and in not one of the places has the Labour party actually gone up from their previous contestation percentage.

“Of course, Labour entered 2017 in a very bad position and had done even worse in the local elections, and then did remarkably well in the general election. I think what it actually shows is that the love affair with Jeremy Corbyn of 2017 is gone, and people are taking a different judgment of the Labour party.

“Council byelections produce some very weird results. There’s always local factors. But on the range of seats we’re talking about, it really does indicate the scale of the volatility of voters in general, and in particular when looking at the Labour party.”