LONDON -- British Prime Minister Keir Starmer warned on Wednesday that a leadership challenge against him would plunge the Labour Party into "chaos", as speculation mounted over a possible move to replace him following the party's poor local election results.

Speaking amid growing unrest within Labour, Starmer urged ministers not to destabilize the party further. A leadership challenge on Thursday "would 100 percent do that", Sky News quoted him as saying.

The warning came as British media reported that Health Secretary Wes Streeting is considering resigning and launching a leadership challenge as early as Thursday morning, which means he would require the backing of 81 Labour members of parliament, including the candidate.

Streeting was seen walking into No 10 Downing Street on Wednesday morning before the state opening of Parliament, though he left less than 20 minutes later.

Pressure on Starmer has intensified since Labour's disappointing showing in last week's local elections. Several junior government ministers resigned on Tuesday, while several cabinet ministers publicly reaffirmed their support for the prime minister.

Meanwhile, 11 Labour-affiliated unions issued a joint statement calling for the party to prepare for a leadership transition. "It's clear that the Prime Minister will not lead Labour into the next election," it said, adding that "at some stage, a plan will have to be put in place for the election of a new Leader."

Also on Wednesday, the Scottish National Party threatened to table a motion of no confidence against Starmer, saying that the "leadership circus can't go on any longer," according to Sky News.