Brexit Party leader Nigel Farage has said his party will not contest the 317 seats won by the Conservative Party in the 2017 election, but would contest nearly all other seats.

In a significant boost for British Prime Minister Boris Johnson ahead of a 12 December election, Mr Farage said he did not want anti-Brexit parties to win the election so he was standing down candidates in 317 of the 650 seats up for grabs.

"The Brexit Party will not contest the 317 seats the Conservatives won at the last election," Mr Farage said, adding that he had made the decision overnight.

A week ago, he promised to field 600 candidates.

Mr Farage cast his move as a step that would prevent another EU referendum and make a hung parliament much less likely.

He said Mr Johnson was proposing a Brexit that sounded like the option that the British people had voted for in a referendum of EU membership three years ago.

Mr Farage said: "But what we will do is concentrate our total effort into all of the seats that are held by the Labour Party, who have completely broken their manifesto in 2017.

"Our action prevents a second referendum from happening. We will also take on the rest of the Remainer parties."

Mr Johnson said he had "absolutely not" called Mr Farage to agree a deal.

He told broadcasters: "I'm glad that there's a recognition that there's only one way to get Brexit done and that's to vote for the Conservatives." 

Read more: A win for Johnson as Farage blinks first

Sterling leapt by more than half a cent against the US dollar, which hit a high of $1.2890, as Mr Farage spoke and British government bond prices fell.

"It is obviously good for the Conservatives mainly because we know the Brexit Party is mainly threatening the Conservative vote," said Sara Hobolt, a professor who specialises in polling at the London School of Economics.

"Farage is sending a signal saying Boris Johnson represents the real Brexit vote and that could matter in the seats where he is standing," Ms Hobolt said.

"If I was Boris Johnson I would be pretty pleased by this."