Welsh Labour MP Jo Stevens is backing Owen Smith in the Labour leadership contest.
The Cardiff Central MP had been considered as one of Jeremy Corbyn's most loyal supporters - she was one of 40 MPs who refused to back the no confidence motion in their leader.
But she has now told supporters she will back the Pontypridd MP in the leadership contest.
In an email to party members, Ms Stevens wrote: "During the past two weeks it has become painfully obvious that we have been unable to fulfil the very basic day to day operation as the official opposition in Parliament.
"We cannot present ourselves as a government in waiting without leadership and a leadership team that commands the respect and support of not only members....but Labour voters and potential Labour voters."
A vigil has been held on the steps of the Senedd in Cardiff Bay following the tragedy in Nice last night. At least 84 people have been killed after a lorry was driven into crowds celebrating Bastille Day.
Speaking at the vigil, Marie Brousseau-Navarro, the honorary consul of France in Cardiff, said it was comforting to see Wales show solidarity in wake of the terror attacks in Nice.
A Welsh journalist, working in France, has told ITV Cymru Wales News, that most people in France have been expecting another terror attack.
Most people in France have been expecting some kind of attack at some point, the only real surprise was that it was outside paris.
We're all aware that another attack will happen and it could come in any time and any place, everyone always expects the capital to be struck.
A Welsh man, originally from Ferryside, who now lives in Nice, has told ITV Cymru Wales News, that the 'whole city is on a knife edge'.
Edward Jenkins says he fears that he will know some of the victims of the Bastille Day attack.
The whole city is on a knife edge.
There's a terrible underlying fear of dread that you're going to see someone you know on a list of people who have been killed.
Mr. Jenkins says that the city has been on high alert for a while now, since the Sousse attack in Tunisia.
Wales First Minister has described the Bastille Day lorry attack in Nice as 'shocking and horrific'.
In an interview with ITV Cymru Wales News, Carwyn Jones has said that the people of Wales stand shoulder to shoulder with the people of France.
Shocking and horrific are the words that I would use.
The people of Wales stand shoulder to shoulder this afternoon - not just in sympathy, but in solidarity as well.
We cannot allow attacks like this to change our free society and the way that we live.
A vigil will be held this evening at the Senedd to pay tribute to victims of the Bastille Day lorry attack in Nice.
At least 84 people have been killed - several of them children - after a lorry ploughed into crowds.
Flags at the National Assembly for Wales have been lowered to half mast following the Bastille Day lorry attack in Nice.
The vigil will take place at the steps of the Senedd from 5pm.
It is being organised by the French Consular Association in Wales.
First Minister Carwyn Jones tweeted saying, "Truly shocked at the appalling events in Nice. My thoughts are with all the victims and their families involved in this horrific attack."
Truly shocked at the appalling events in Nice. My thoughts are with all the victims and their families involved in this horrific attack
The Tour de France's 13th stage went ahead under tightened security on Friday following the deadly attack in Nice which has left at least 84 dead.
Riders observed a minute's silence in tribute to the scores of victims of the Bastille Day attack, which President François Hollande called a terrorist act.
The stage began at 08.05 BST, later than scheduled, after race organisers held a crisis meeting with the prefect of the Ardeche department and the gendarmerie - France's military force tasked with police duties.
Race Director Christian Prudhomme said: "We want this day to be a day of dignity as a tribute to the victims. We asked ourselves [whether the stage should be cancelled] but we think, after agreeing with authorities, that the race must continue."
France must "learn to live with terrorism", the country's prime minister has said, as he announced three days of national mourning beginning on Saturday
“Terrorism is a threat that is weighing heavily on France,” Manuel Valls said.
"We’re faced with a war that terrorism has brought against us. The goal of the terrorists is to make us scared. We won’t give in to the terrorist threat, we must stand together, united. France has been once again struck in our flesh.
"Times have changed and we should learn to live with terrorism. We have to show solidarity and collective calm. France has been hit in its soul on the 14 July, our national day.
“They wanted to attack the unity of the French nation. The only dignified response is that France will remain loyal to the spirit of the 14 July and its values.”
The deadly Bastille Day lorry attack in Nice is a reminder of the "active terrorist threat" facing Europe, Europol's director has said.
Rob Wainwright added: "Europol stands in full solidarity with the citizens of France, as it does with all victims of terrorism, and has made available to the French authorities the full operational capabilities of the European Counter Terrorism Centre."