Owen Smith has put his campaign launch on hold after the attacks in Nice last night, which left more than 80 people dead and many more injured.
The Pontypridd MP had planned to set out his vision for Labour and why he wanted to challenge incumbent Jeremy Corbyn, but postponed the launch after what he called the "heartbreaking" news from France.
Read more: Smith says UK should vote on Brexit again
Last night a lorry bulldozed through a crowd in the southern French city of Nice, taking the lives of 84 people, including children. The driver carried on for 2km through a crowd that was celebrating Bastille Day.
President Francois Hollande said the attack was of "an undeniable terrorist nature".
Smith had planned to say that he could secure Labour's future, while Corbyn fights on despite a vote of no confidence from Labour MPs late last month.
Rival Angela Eagle, former shadow business secretary, launched her bid earlier this week.