The results saw the Brexit-championing UK Independence Party fail to capitalise on the June referendum vote to leave the European Union.
But while Labour celebrated fending off UKIP's bid to replace them in their traditional post-industrial heartlands, the historic loss elsewhere of a seat to the Conservatives was a big blow to their prospects in the 2020 general election.
In yesterday's two parliamentary by-elections, Labour clung on to its Stoke-on-Trent Central seat, in the British city that registered the highest vote for leaving the EU.
Both parties had ploughed huge resources into the contest in west central England, which had been pitched as make-or-break for both sides.
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Nuttall insisted the defeat did not mark the end of UKIP.
"This seat was 72 on our target list. There's a lot more to come from us," he said.
"There are other issues beyond Brexit."
But Labour's relief at retaining Stoke-on-Trent Central could not mask the impact of their loss of the rural seat of Copeland on the northwest English coast.
It was the first time a governing party has taken an opposition-held seat in a by-election since 1982.
Many jobs in the constituency depend on the Sellafield nuclear processing facility, and veteran leftist Labour leader Jeremy Corbyn's strong anti-nuclear stance may have damaged the party's fortunes.
"Labour's victory in Stoke is a decisive rejection of UKIP's politics," Corbyn wrote on Twitter.
"But our message was not enough to win through in Copeland.
"To win power to rebuild and transform Britain, Labour will go further to reconnect with voters and break with the failed political consensus."
Nationwide opinion polls keep May's centre-right Conservatives 18 points ahead and bookmakers lengthened the odds on Labour winning the 2020 general election after the Copeland defeat.
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