Gale-force winds and heavy rain could have a dramatic effect on turnout in the Copeland and Stoke byelections, Labour sources have said, with some fearing any negative effect could hand victory to the Conservatives in Cumbria.

The Met Office is predicting significant disruption with an amber warning issued for a large band of the country stretching from northern Wales to Norfolk, where gusts of up to 80mph are forecast on Thursday.

Some Labour MPs said they were becoming increasingly concerned about the effects of Storm Doris on turnout in Copeland and Stoke and those that had visited the seats said the Tory campaigns in both were growing in confidence.

“Turnout is going to be the problem for us. I think people on a cold day in February will think: ‘Oh, why should I bother?’” one Labour MP said. “The Tories were only a handful of votes behind Ukip and I think they could even win in Stoke. If you’re an anti-Labour voter, and they are the majority in Stoke, you could decide to vote Tory. It could happen with a low turnout.”

On the doorstep in Copeland, where Labour has a slim 2,564 majority, there was little enthusiasm to turn out and vote even among loyal Labour voters, the MP said.

“People are just not bothered about whether the seat stays Labour,” they said. “Jeremy is definitely a factor and his very late delivery of a guarantee on Moorside was too late. Given his past record on nuclear power and on Trident, just down the road, it makes it too much for people to believe.

“There is a general sense of disillusionment, and the most frequent response is: ‘I don’t know who to vote for’ – and then it means people are far more likely to stay at home.”

However, another Labour source said a low turnout did not necessarily mean there would be a Conservative victory in Cumbria.

“The Tories need people to turn out just as much as we do,” the source said. “We have a very strong ground operation up there. It is genuinely impressive and disciplined considering the geography. The messaging is, simply, we are committed to nuclear power and focused on saving the maternity services.”

Another Labour backbencher believed the party’s relentless focus on the NHS would see them clinch Copeland, saying: “I am getting the sense that the NHS has really punched through, as indeed it should. Campaign issues-wise, this is almost the perfect byelection for us.”

The exact timing and course of Storm Doris remains uncertain, the Met Office said, but it is forecasting gale-force winds late on Thursday morning. The warning states: “There is potential for a short-lived core of very strong winds to develop. While some disruption is expected outside of this stronger swath of winds, the amber area looks most likely to see more significant disruption.”