UK Prime Minister Boris Johnson's Conservative Party has suffered a surprise defeat in a parliamentary by-election.

Voters in the affluent constituency of Chesham and Amersham, a few miles to the northwest of London, had comfortably elected a Conservative Member of Parliament at every election since 1974.

And in national opinion polls, and the most recent local elections, the Conservatives had been showing strongly against the main opposition, the Labour Party, especially in Labour's "red wall" northern heartlands.

But in the by-election prompted by the death of long-standing Conservative MP Cheryl Gillan, voters turned to the fourth party in the UK Parliament – the Liberal Democrats' candidate Sarah Green – who won with a majority of 8,028.

READ MORE:

France ends outdoor face mask rule

Sending an airship to Mars?

Artists getting tunes out of trash

Asked about the result, Conservative Minister of State Kit Malthouse told Sky News: "We would have hoped for a better results ... it's tough and disappointing."

Johnson's parliamentary seat is just 10 miles away, in West London. Some have warned that his drive to win ground in traditionally Labour areas could lead to disenchantment in his party's traditional "blue wall" heartlands in central and southern England.

Ed Davey, the leader of the Liberal Democrats – which is the most pro-EU of the UK-wide parties –  said voters "have been taken for granted, they feel that the Conservative Party isn't listening to them."

He added that "if the Lib Dems can beat the Conservatives here, we can beat them anywhere. The blue wall can be smashed."

Gillan had represented the seat since 1992. At the last election in 2019, Gillan won with a majority of 16,223 votes. 

It is not clear how much national impact the result will have, as by-elections can often provide very different results from national elections.

In this case, the main issue was widely seen to have been the construction of the HS2 high-speed rail line, which goes through the constituency, as well as fears about the impact of new planning laws on what is currently protected undeveloped land.