By Darren Staples and Phil Noble
STOKE-ON-TRENT/WHITEHAVEN, England (Reuters) - British Prime Minister Theresa May's Conservatives secured a landmark victory in a parliamentary by-election on Friday, strengthening her hand ahead of Brexit negotiations as her rivals suffered damaging setbacks.
The Conservatives captured the northwestern seat of Copeland that Labour have held since 1935, the first by-election gain for a governing party for 35 years and a result that piles pressure on the opposition's under-fire socialist leader Jeremy Corbyn.
In the central English seat of Stoke-on-Trent, Paul Nuttall, leader of the populist anti-EU UK Independence Party, failed to overturn a Labour majority despite the fact almost 70 percent of the city's voters backed leaving the European Union at last year's referendum. That cast doubt on his future too.
The Conservatives also increased their share of the vote in Stoke from the 2015 election.
The two results point to May's tightening grip on political power following the Brexit vote, and will be used as evidence that her strategy of pursuing a clean break with the EU has stemmed rising right-wing populism without denting her ability to take votes from an increasingly left-wing Labour Party.
Although Labour avoided the worst-case scenario of two defeats, Corbyn is likely to face renewed criticism.
"The result ... is a disaster for us. We should not try to insult people's intelligence by suggesting other than that," Labour lawmaker John Woodcock told BBC radio.
In an email to party supporters, Conservative Chairman Patrick McLoughlin said the result was "an important show of support" for May's Brexit plans.
May's spokesman said the British leader, who took over following last year's Brexit vote, did not have any plans to hold an early national election.
Many Labour parliamentarians worry Corbyn's leadership is damaging their fight for a "softer" Brexit, with closer ties to the EU's single market, while his leftist agenda is making the party unelectable before the next national election in 2020.
"We’re actually on course to an historic and catastrophic defeat," Woodcock said.
"This is a time where the country really needs an effective opposition and they need an alternative to what I think is a very damaging approach the Conservative government is taking on the issue of leaving the European Union."
Despite long-running unrest within the upper ranks of his party, Corbyn is unlikely to face a fresh leadership challenge because he retains strong support among the grassroots Labour members who re-elected him last year after a botched coup.
Corbyn told reporters he was disappointed with the result, but asked if he was considering stepping down, he said: "No, I was elected to lead this party, I am proud to lead this party."
In Stoke, UKIP were unable to capitalize on the anti-establishment sentiment it tapped so successfully when persuading voters to leave the EU last year, despite pouring resources into a campaign waged on fertile Brexit territory.
The party's former leader and best-known figure Nigel Farage had warned last week the Stoke vote was vital for the future of the party, beset by in-fighting since the Brexit vote.
"UKIP's time will come ... there's a lot more to come from us, we're not going anywhere, I'm not going anywhere," Nuttall told reporters after the result.
A campaign dogged by personal gaffes risks reigniting feuding within the party, which has struggled to break into mainstream politics despite playing a pivotal role in forcing the government to call an EU referendum and the subsequent campaign to leave the bloc.
"If you want to be an electoral force and a political force in this country you have to be able to win by-elections in difficult circumstances as a third party," Rob Ford, a politics professor from Manchester University and an expert on UKIP, told Reuters.
"If they can't do that even in the most propitious of seats, people will start to have questions."
(Reporting by William James and Michael Holden, Additional reporting by Kylie MacLellan; Editing by Guy Faulconbridge and Tom Heneghan)