Former prime minister Gordon Brown says he’s “not afraid” of another referendum on Scottish independence, though he does not believe one will take place soon.

As he launched a renewed campaign to keep Scotland in the UK, he announced former Scottish Green Party co-convener Robin Harper would be on one of his organisation’s commissions.

Mr Brown’s Our Scottish Future think tank will become a “campaigning movement” which seeks to appeal to “middle Scotland” – those who are not entrenched in their positions on the union or independence.

The group is setting up a number of commissions which will examine how greater co-operation around the UK will benefit Scotland on issues such as health, poverty, the environment, and jobs.

The environment commission will be chaired by Mr Harper, who became the Greens’ first elected parliamentarian in 1999.

Cancer specialist, Prof David Kerr, will chair the health commission and Professor Ronnie MacDonald of Glasgow University will examine the economy along with Eddie Barnes.

Mr Brown, who played a key role in the No campaign during the 2014 vote, said those in middle Scotland are “patriots not nationalists” who want to see greater cooperation between the UK’s governments.

Speaking to journalists on Monday, he said he did not believe any request for indyref2 would come for months if not longer.

On the legal position of another referendum, he said: “Nicola Sturgeon should commission and publish the legal advice the Lord Advocate can give.

“And the UK Government should publish their legal advice.

“In an open, transparent democracy that legal advice should be on the table, and that should tell us whether Nicola Sturgeon is planning a wildcat referendum, or a Catalonia-style referendum.”

He called on the Prime Minister to set up a permanent forum of the UK’s nations and regions to discuss the way forward on a number of issues.

Mr Brown also claimed there were currently no answers to the question of “what does nationalism actually mean?”

Asked if he would be a figurehead in a pro-Union campaign in any future referendum, he said he had no aspiration to lead anything.

The former prime minister said: “I’m not afraid of the referendum. I actually think that we’ve got all the arguments and will develop all the arguments that will win that referendum.

“So I believe if there was a referendum we’d win it.

“But the truth is that I don’t know what’s going to happen in the next few years.

“I’m saying – get round a table. You’ve got a political promise that the SNP made, but a legal dubiety.

“And you’ve got a legal reality that the UK Government claims. That cannot be solved in the courts of law.”

He said speculation about any court case was currently “irrelevant” and the focus should be on how to increase co-operation in the UK.

Ms Sturgeon, the Scottish First Minister, has already told the Prime Minister that the Holyrood election result means that “question of a referendum is now a matter of when – not if”.

Mr Brown said that Our Scottish Future would argue for a “reformed” United Kingdom, as he called for a change of tactics from the Prime Minister.

He said: “Boris Johnson may believe that he can both save the union and be an English nationalist.

“But no Prime Minister can hold the UK together if they are at war with a large part of it.”

Mr Harper, the Scottish Green Party’s first MSP, said he was “delighted and honoured to come on board with Our Scottish Future and to take part in this necessary and worthwhile enterprise”.