SCOTTISH Labour deputy leader Anas Sarwar has been sidelined from his party's referendum campaign following a power struggle with Johann Lamont, according to a senior party source.

This newspaper has been told that Lamont recently seized control of her party's pro-Union campaign from Sarwar, who is now in charge of the Labour battle bus.

The launch of Better Together as the formal pro-UK campaign body was awkward for some Labour members who did not want to stand alongside the Tories.

Sarwar, the Glasgow Central MP who was elected Lamont's deputy in 2011, was a key figure behind the creation of United with Labour (UWL), the party's separate campaign to keep Scotland in the Union.

The ambitious MP was the UWS standard-bearer and explained his role during his March conference speech: "I want to thank Johann for giving me the honour of leading this campaign for Labour."

However, last month the Sunday Herald learned that Sarwar had been elbowed aside to make way for Lamont, who is now UWL's key figure. Relations between the leadership duo are believed to have taken a dip.

Scottish Labour's press release last week on UWL's formal campaign launch confirmed the change of emphasis.

Although Sarwar described himself as UWL leader in March, the release noted that "Johann and Gordon [Brown]" would hold a press conference on the launch.

On the evening rally, it stated: "Johann Lamont will launch the Labour campaign to keep Scotland strong in the United Kingdom at a rally in Glasgow."

The release added that Lamont would be "joined" by Brown, Sarwar and Glasgow City Council leader Gordon Matheson.

Lamont and Brown had the prime speaking slots at the Glasgow rally, with Sarwar getting a few minutes at the end.

Sarwar has now gone from leading the campaign to taking charge of a bus.

A Labour press release from Wednesday, the day after the rally, stated: "Led by Anas Sarwar, the 'Indy Ref Express' will take volunteers all over Scotland to campaign for a No vote in the referendum."

He is also in charge of the party's "Truth Team", which aims to expose holes in the SNP's independence case.

A senior party insider said: "It's an open secret that Anas has been pushed to one side, as Johann and her people seek to take control of the referendum campaign. It's damaging for Anas politically, but it has the potential to be more damaging for Labour's campaign. And just when Scotland faces the biggest decision in its history, Labour decides to look inwards."

The episode is also said to reflect wider tensions at the top of Scottish Labour.

Senior party insiders who are closer to Sarwar than Lamont are speculating about whether she wants to lead the party into the 2016 Holyrood election.

Ian Price, the party general secretary and a key Lamont ally, has also become a target for internal Labour sniping.

SNP MSP James Dornan said: "The No campaign can shove Mr Sarwar on a bus, but a fundamental problem they have is that all of their leaders are unpopular. They have a problem with both their message and their messengers.

"As the latest poll shows, person for person, whether it is Alex Salmond against David Cameron, Nicola Sturgeon versus Johann Lamont, or Patrick Harvie and Willie Rennie, the Yes leaders all have positive ratings because they are trusted, and the No leaders' ratings are all negative."

A Scottish Labour spokesperson said: "After our launch this week, Scottish Labour is in full campaign mode and is taking our positive message to every part of the country.

"Johann Lamont will continue to lead the campaign and will be supported by former prime minister Gordon Brown who is doing rallies all over the country.

"Her deputy Anas is the referendum co-ordinator and will play a pivotal role, leading campaign teams all round the country and speaking at events."

A spokesman for Sarwar said: "Having spoken at our launch rally in Glasgow on Tuesday, unveiled the referendum battle bus before speaking at another rally in Dundee on Wednesday, signed off a million leaflets on Thursday and set up Labour's Truth Team on Friday, Anas is working hard to secure a No vote on September 18 and getting on with the referendum co-ordinator job he was asked by Johann to do."