Angela Eagle is the other MP trying to wrest the party leadership from the veteran socialist
A second candidate joined the race on Wednesday to try to unseat Britain's opposition Labour leader Jeremy Corbyn, who is battling a party revolt in the wake of the Brexit vote.
"I will stand in this election and I will do the decent thing and fight Jeremy Corbyn on the issues," Labour lawmaker Owen Smith told the BBC.
He will join fellow MP Angela Eagle in trying to wrest the party leadership from the veteran socialist, who has refused to quit despite a major rebellion by his MPs.
The winner of the contest, which will formally get underway with an announcement of the timetable on Thursday, is expected to be crowned in September.
Smith said he had decided to stand after seeing a "dramatic collapse of faith and confidence in Jeremy" over the last couple of weeks.
Many moderate Labour MPs have never reconciled themselves to Corbyn's election as leader last September secured thanks to strong support among ordinary party members.
They moved against him in the wake of Britain's shock June 23 vote to leave the European Union -- an outcome deplored by most of the parliamentary party.
Three-quarters of Labour MPs backed a vote of no confidence in Corbyn on June 28, accusing him of lacklustre leadership in the campaign which culminated with many longtime Labour voters in underprivileged areas defying the party line and backing Brexit.
Many party grandees also fear he would be unable to win a general election if one were called early, although incoming prime minister Theresa May, who takes over from David Cameron later Wednesday, has ruled out an early vote.
Late Tuesday, Corbyn won a first victory over his critics after the party's executive committee ruled he would automatically be included on the leadership ballot.
The decision means that -- unlike his challengers -- he does not need the required 51 nominations from Labour MPs or members of the European Parliament (MEPs) to stand.
Smith, a former BBC radio producer seen as more centrist than Corbyn, has only been a member of parliament since 2010, representing the Welsh constituency of Pontypridd.
But the 46-year-old has been a member of the Labour party since he was 16, and was special advisor to Paul Murphy, the Labour government minister in charge of Wales and then Northern Ireland, between 2002 and 2005.
After a stint working as a media advisor to pharmaceutical group Pfizer, he became an MP when Labour moved into opposition and became its spokesman on Welsh affairs.
He appeared in a list of possible candidates to replace outgoing leader Ed Miliband following Labour's second successive general election defeat last year.
But he did not step forward then and was named shadow work and pensions minister under Corbyn in September 2015.