Jim Murphy was last night tasked by Gordon Brown with taking the fight to the SNP in Scotland, after Des Browne stepped down as Scottish Secretary.

Mr Browne, holding both the defence and Scotland posts in the Cabinet, said he had told the Prime Minister "some months ago" that he was looking to step down.

He was rumoured some months ago to be expressing a wish to spend more time with his family, after a gruelling period carrying two jobs at Westminster.

But in the short-term Mr Browne was at pains to demonstrate his loyalty to the Prime Minister: "I fully understand why he has made the decisions that he has in this reshuffle. It has been a privilege to serve this Labour government in all six of the ministerial jobs that I have held over the last seven years."

He added: "I am proud of what I have achieved as Scotland's voice in the Cabinet and in renewing the relevance of the Scotland Office."

He cited legislation giving legal protection for Scotch whisky and establishing the Calman Commission on devolution as examples of tangible benefits for Scotland.

Mr Browne had the difficult task of being the government's representative in Scotland at a time when the SNP were enjoying a honeymoon. The Scottish Secretary adopted a relatively non-combative stance, seeking to undermine Mr Salmond quietly rather than head-on.

He also had to do the Dover House job at a time when, as Defence Secretary, he was in charge of over-stretched armed forces, facing war on two fronts in Iraq and in Afghanistan.

Mr Browne's successor moves to the Scotland Office from the post of Minister for Europe, a portfolio he was believed to have fought hard to retain.

Mr Murphy, 41, entered parliament in the Labour landslide of 1997, where he captured what was then the seat of Eastwood, at that point the Tories' safest in Scotland.

A former student leader, Mr Murphy was seen almost as much of a Blairite as Peter Mandelson. He was a key player in The Network, the internal grouping which paved the way for the New Labour reforms.

The East Renfrewshire MP is likely to adopt a more combative approach to the Salmond administration. While this will delight many Scottish Labour MPs it remains to be seen how it will play with voters generally.

The first big test will come in the Glenrothes by-election in a few weeks' time. It was reported yesterday that Gordon Brown had decided to set aside the convention he cited for not campaigning in Glasgow East, and to feature personally in the Glenrothes campaign.

Mr Murphy is bound to play a major role in the campaign, which means that if the SNP overturn Labour's majority of more than 10,000 it will inflict grievous damage on both Mr Brown and Mr Murphy.

The new Scottish Secretary will also have to decide how to handle broader issues such as the Calman process, the expected review of the Barnett formula, and the future of council tax benefit if local government finance is reformed in Scotland.