In a tactical retreat that could have important ramifications, Nigel Farage, the leader of Britain’s populist Brexit Party, on Monday promised not to run candidates in areas held by the ruling Conservatives. The move was expected to bolster Prime Minister Boris Johnson’s prospects in a topsy-turvy general election next month.

Mr. Farage had been under pressure from his own supporters who worried that, by splitting the pro-Brexit vote, he might deprive Mr. Johnson of the victory he needs to pull Britain quickly out of the European Union.

However, the Brexit Party is still planning to fight seats currently held by the opposition Labour Party, and in doing so could draw away Conservative voters in the electoral battlegrounds of the middle and north of England. The Conservatives desperately need to grab Labour seats in areas there that voted Leave in the 2016 referendum on whether to remain in the European Union.

So, while the partial pact helps the Conservatives, analysts do not think it will do so enough to decide the outcome of an election likely to determine whether Britain leaves the European Union at the end of January, as Mr. Johnson wishes, or holds another Brexit referendum next year.