Bob Ainsworth was named Britain's new defence secretary in a cabinet reshuffle on Friday after the incumbent John Hutton announced his resignation.
Prime Minister Gordon Brown paid tribute to Ainsworth, who was armed forces minister before the reshuffle, effectively the number two at the defence ministry.
Brown said Ainsworth would make an "excellent" defence minister and was already "very popular with the generals".
Ainsworth takes over the role at a critical time for Britain's armed forces which are battling Taliban insurgents in Afghanistan.
Britain is the second-biggest contributor of foreign troops to Afghanistan after the United States, deploying around 8,300 as part of a NATO-led force based mostly in the south, the heartland of a Taliban insurgency. Britain last month announced a temporary increase to 9,000 soldiers, to help provide security for elections in August.
Ainsworth will accompany Brown to France for D-Day commemorations Saturday, the prime minister said.
Brown said Hutton had informed him "some time ago" that he wanted to resign for family and personal reasons. "I want to pay tribute to what he has achieved," Brown said.
Brown announced the cabinet reshuffle in an attempt to reassert his authority in the wake of a string of resignations from ministers and amid a public furore over expenses claims by lawmakers.