Kevan Jones is standing down as shadow defence minister, saying he disagrees with Labour leader Jeremy Corbyn on scrapping Trident.
Mr Jones is MP for the North Durham constituency, which he has represented since 2005.
He said he "respected" Mr Corbyn's view that Britain's nuclear deterrent should be scrapped, but disagreed and could no longer remain as shadow defence minister.
"I am determined to continue making the case for Labour to have a strong, credible defence and security policies, and I feel that the most effective way for me to continue to do this is from the backbenches," he wrote in a resignation letter.
He became the third front bencher to resign today after Mr Corbyn sacked two "disloyal" senior figures and promoted a Trident opponent in his so-called "revenge reshuffle".
Mr Jones, Jonathan Reynolds and Stephen Doughty quit within the space of 90 minutes, citing differences with their leader on key policy issues and the treatment of their colleagues.
Mr Jones' constituency includes the County Durham towns of Chester-le-Street, Sacriston and Stanley.
Before becoming an MP, he was a Newcastle city councillor from 1990 to 2001 and an elected officer of the GMB union.