The leader of the most prominent pro-UK campaign group has revealed he has quit Scottish Labour after becoming disenchanted with Jeremy Corbyn.

Ex-MSP Graeme Pearson, who is the chief executive of Scotland in Union (SIU), said Corbyn was not having an impact and questioned whether he could lead Labour into Government.

He also appeared to damn Scottish Labour leader Kezia Dugdale with faint praise by saying she was doing “all right” in the job.

Pearson was a senior police officer and the director-general of the Scottish Crime and Drug Enforcement Agency before getting elected as a Labour MSP in 2011.

The former police officer held shadow cabinet positions when Johann Lamont and Jim Murphy led Scottish Labour, but he retired from Holyrood at the last election.

In January, he was unveiled as the chief executive of SIU, a non-party organisation committed to making the “positive case for Scotland staying in the UK” and opposing a second referendum.

Pearson launched the group’s ‘Project Listen’ in Glasgow’s Lighthouse last week, with the aim of signing up a quarter of a million supporters by the summer.

However, it has now emerged that he stopped listening to his own party some time ago.

Speaking to the Sunday Herald, Pearson, who was on the moderate wing of Scottish Labour, said: “I am not a Labour member.”

He said his decision to quit was “not solely about Scotland in Union, but that was one of the benefits that would allow me to be non-partisan.”

Pearson explained: “I felt that my time in the Labour party had come to a natural conclusion and that the Labour party was going in a direction that wasn’t important to me personally.”

He confirmed Corbyn’s leadership, which has seen the party hit record lows in the opinion polls, was a factor behind him quitting: “I’ve got every liking for the man as an individual, but I just found the direction he was going in, and the way in which he conducted politics, wasn’t influential and didn’t have impact on the big issues of the day.”

He said: “It’s a big difference between [being] a thoroughly decent guy to be with, and somebody who would lead a party and deliver for ordinary people. I know he understands some of the issues that affect ordinary people, I just didn’t think he would deliver on the outcome of leading a Government.”

He added: “My membership was due to lapse in January and I had decided to allow my membership to lapse. And I began with Scotland in Union in January. The cynics will say ‘one led to another’, but it didn’t.”

Pearson backed Ken Macintosh over Dugdale during the last Scottish Labour leadership contest. Asked how he believed she was doing in the job, he said: “I think she’s doing all right. I think that she plugs away.”

He said Scottish Labour figures were working “very hard”, but noted: “Until and unless this overbearing nationalist pressure is challenged in a forthright way and overcome, it is always going to be difficult for Labour.”

Pearson's resignation is the latest sign that Corbyn's leadership is dividing Scottish Labour.

Senior party figures were angry with Corbyn last week after he said he would be "fine" with a second independence referendum, a view that put him at odds with Dugdale.

A Scottish Labour spokesman said: "We thank Graeme for his years of service to the party and wish him well in his new independent role."

An SNP spokespersons said: "It's not a huge surprise that Graeme Pearson and his band of ultra-unionists are uncomfortable with Jeremy Corbyn, who said that an independence referendum would be 'absolutely fine'.

"But deep down we know that Kezia Dugdale is just as unhappy with Jeremy Corbyn and believes he is completely unelectable. She's said as much.

"That leaves Labour telling the people of Scotland they should be stuck in a union where they think only the Tories can win - a hard case for anybody to sell."