Angela Eagle, a senior lawmaker from Britain's main opposition Labour Party, said she would make a bid to replace Jeremy Corbyn as party leader.
Labour has been engulfed in a deep crisis since Britain's shock vote to leave the European Union on June 23, with Corbyn clinging on despite mass defections from his policy team and a motion of no confidence backed by most members of the parliamentary party.
"On Monday morning I will announce my candidature for leader of the Labour Party. I will explain my vision for the country and the difference a strong Labour Party can make," Eagle said in a statement.
I will explain my vision for the country and the difference a strong Labour Party can make
She said Corbyn had failed to lead Labour lawmakers as an organised and effective force to hold the Conservative government to account and demonstrate they were ready to form a government in the event of an early national election.
Discontent with Corbyn, who spent three decades on the hard-left fringe of the party before a wave of grassroots enthusiasm swept him to a surprise victory in a leadership contest last summer, has been simmering for months among Labour lawmakers.
Many were unconvinced by the veteran socialist's leadership ability and their misgivings exploded into open crisis after the EU referendum.
The party's official stance had been to support staying in the EU, but critics accused Corbyn of campaigning so half-heartedly that many Labour voters did not know what the party's position was ahead of the referendum.
Corbyn has repeatedly said he has no intention whatsoever of standing down, citing his overwhelming victory in the contest decided by grassroots members last summer. He is expected to stand again in any leadership contest.
Eagle was business and trade policy chief in Corbyn's team until she resigned on June 27 as part of a wave of resignations that plunged Labour into disarray.
The chaos in the party has added to a sense of instability in British politics since the Brexit vote, with the Conservatives also rudderless after Prime Minister David Cameron announced the morning after the referendum that he would resign.
Two women, interior minister Theresa May and junior energy minister Andrea Leadsom, are competing to replace Cameron as leader of the Conservative Party and as prime minister. The victor is expected to be announced on September 9.