Jim Murphy has resigned from the shadow cabinet to focus on his campaign to become the next Scottish Labour leader.
Mr Murphy had been the shadow international development secretary since October of last year.
He is one of three candidates standing in the contest to replace Johann Lamont, who quit as leader of the Scottish Labour Party last month.
Meanwhile, the MSP Kezia Dugdale has confirmed she has entered the deputy leadership race.
Speaking to BBC 5Live's Pienaar's Politics programme, Mr Murphy said: "I want to dedicate myself full-time to changing Scotland and changing the Scottish Labour Party.
"It's difficult to do that and to serve in the Shadow Cabinet, and therefore after talking to Ed Miliband over the past couple of days I've decided to stand down from the shadow cabinet."
Confirming her deputy leadership candidacy on Twitter, Ms Dugdale said: "Delighted to have secured enough nominations for deputy leader to be on ballot - will be saying much more tomorrow".
She is regarded as being a rising star within Scottish Labour, and had been seen as a potential candidate for the party leadership before ruling herself out soon after Ms Lamont quit.
The Lothians MSP is currently Labour's education spokeswoman in the Scottish Parliament.
Seven of the 11 MPs and MSPs to have nominated her for deputy leader have also nominated Mr Murphy for the leadership job.
The only other candidate to have confirmed she is standing to replace Anas Sarwar as deputy leader is the North Ayrshire and Arran MP Katy Clark.
Ms Clark has received the support of the Unison union, which on Saturday backed the MSP Neil Findlay to be party leader.
The Transport Salaried Staffs' Association (TSSA) and Bakers Food and Allied Workers Union (BFAWU) have also said they are supporting Ms Clark.
She said: "Thousands of trade unionists will be participating in this election which is a real opportunity for the leadership and deputy leadership candidates to engage with the thousands of trade unionists up and down the country who will have a vote on who leads Scottish Labour.
"Trade union members and Labour Party members have a shared agenda of wanting a Scottish Labour Party which fights to defend and improve public services and living standards.
"Scottish Labour needs to listen to what trade union members are saying. I hope that this contest will see a genuine debate as to how we can build stronger links between Scottish Labour and the trade union movement and trade union members."
Unison Scotland Labour Link chairman Gordon McKay said: "Katy Clark has a great record of campaigning on the issues that matter to Unison members in Scotland. On public services, employment rights, pensions, inequality and many other concerns.
"Her views have remained consistent with Labour's traditions and values and that is vital if Scottish Labour is to build its support in Scotland. Katy has shown before and after becoming an MP that she is on the side of working people."
The MSP Sarah Boyack is standing alongside Mr Murphy and Mr Findlay in the leadership contest. The new leadership team will be named on 13 December.