GORDON Brown is to step out of the shadows to fight for the Union in a series of set-piece speeches during a tour of Scotland.

The former Prime Minister, who has been a semi-reclusive figure on the UK political stage since his humbling fall from power after the 2010 General Election, is to make a high-profile return to frontline politics in the run-up to September's referendum vote.

News of his active involvement in the campaign comes as the chief executive of Shell said he favoured Scotland remaining part of the UK.

Ben van Beurden told a company reception in London that he prized the "continuity and stability" offered by a No vote, in the same way as he favoured the UK remaining part of the EU.

Earlier this year, BP's chief executive, Bob Dudley, said he personally believed Britain "ought to stay together".

In the past four years, Gordon Brown has made only a handful of speeches in the Commons since leaving No 10.

His absence from the regular cut and thrust of the campaign has also been noted. Now the Kirkcaldy and Cowdenbeath MP is to throw himself into it by taking part in a series of hustings.

It will begin with a major speech in Glasgow's east end early next week.

Labour Party sources make clear Mr Brown will not be working under the umbrella of Better Together, the pro-UK campaign led by his former Chancellor Alistair Darling. The pair had a difficult relationship in government, with Mr Darling later revealing how Mr Brown wanted to sack him, but they have now agreed to forget previous disagreements.

Mr Brown will instead be increasing his involvement through the United With Labour campaign.

A source close to the Shadow Cabinet said: "Having a Scottish former Prime Minister of his standing getting involved has got to be significant. His interventions so far have been important, but he now wants to have a big role throughout the campaign."

Mr Brown's colleagues believe that as a politician who is much better regarded north of the Border than south, the former Labour leader will provide the No cause with a major boost and give First Minister Alex Salmond a run for his money.

Sources said Mr Brown's tour would focus mainly on community town halls in what was described as "old-style politics". A Labour MP said: "He will bring gravitas and experience. His record speaks for itself in terms of pensions, tackling poverty, investing in the NHS. Gordon has always been a great believer in how the UK by sharing resources is much bigger than nationalism."

He added: "People listen to Gordon; he is the big beast."

But the ex-PM's political opponents, most notably the Yes campaign, will seek to deride his deeper involvement, stressing how he, and Mr Darling, were at the helm when the economy crashed in the 2008 financial crisis.

In next week's speech, Mr Brown will focus on the argument that Britain is better together in the fight for social and economic justice.

"The choice of where he is doing it is significant," said one senior Westminster source. "It won't be a dry, academic speech, but one set in the communities of Glasgow associated with towering Labour figures like John Wheatley."

Meantime, SNP MP Pete Wishart attacked as "absolutely shocking" the £215,000 spent in the last two years in relation to Mr Brown's public duties as an ex-premier.

But the criticism was denounced as "petty and personal" by a source close to the Labour politician.

An allowance exists to help former PMs fulfil their public duties and £230,000 was spent over the same period in relation to Sir John Major and a similar amount on Tony Blair.

Mr Wishart, who represents Perth, said taxpayers had a right to know what their money was being spent on.

A source stressed Mr Brown was paid only his MP's salary, had refused the prime ministerial pension and gave all the money from his speeches to charity.