Copeland, created in 1983, and its predecessor constituency Whitehaven have returned Labour MPs since 1935.
Mrs Harrison got 44.3% of the vote, increasing the Conservatives’ vote share by more than 8%. She overturned a Labour majority of more than 2,564 to take the seat by 2,147 votes – a swing of more than 6%.
Professor John Curtice, of Strathclyde University, said the Copeland result was the best by-election performance by a governing party in terms of the increase in its share of the vote since January 1966.
It will be taken by her supporters as a vindication of [Prime Minister Theresa May’s] hard-edged drive towards Brexit and her break with the more metropolitan politics of David Cameron.