GORDON Brown today makes his most significant intervention in the ­independence debate to date, saying the Scottish Government's "bogus" claims about oil revenue in an ­independent Scotland would leave a £3 billion black hole.

The former Labour Prime Minister is to claim in a speech that people living north of the Border would face a 10p income tax hike and that Scotland's membership of the EU could cost taxpayers £500 million.

It marks the beginning of a deeper engagement by Mr Brown in the ­referendum campaign, providing the pro-UK cause with a big hitter ­capable of taking on First Minister Alex Salmond.

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Labour sources have been making clear Mr Brown would become more involved in the debate after burying the hatchet with Alistair Darling.

In his address, the MP for Kirkcaldy and Cowdenbeath will acknowledge Scots "want change and not the status quo", explaining that Labour's vision is of "enhanced devolution within the Union" offering the people of Scotland a "fairer deal and a better dividend".

Mr Brown possibly foreshadows, in part, what Scottish Labour's future campaign prospectus will be: ­"maximum devolution of powers" across training, transport, health, the Crown Estate Commission and ­elections; although no mention is made of tax. His detailed proposals will be published in the next month.

He repeats his call, first made last year, for constitutional changes to make the Scottish Parliament ­"irreversible" and for UK legislation to underline the "shared purpose" of the Union to pool resources across its parts.

Praising the "excellent work" of the Better Together campaign, Mr Brown will say one of the main developments in the ­constitutional debate will be the ­recommendations of Labour's ­devolution commission.

"The party that first created a ­powerful Scottish Parliament is best placed to strengthen devolution and to create a stronger Scottish Parliament in a stronger UK," he will say.

"We can show how with our reforms, to be implemented by Labour administrations in Westminster and in Edinburgh, we can address some of the greatest social and economic challenges a future Scotland faces."

Mr Brown will also use his speech to challenge the SNP Government's "bogus and untrue" calculations.

"First," he will tell his audience in Lochgelly Town Hall, "they calculate oil and gas revenues as at least £6.8bn in 2016-17 when all formal and independent forecasts suggest the correct figure is likely to be around £3.5bn, leaving a £3.3bn shortfall. To make this up requires a rise in income tax of 10p."

Secondly, the former PM will claim the Nationalists have failed to calculate the cost of European Union membership without the British rebate, from which Scotland would not benefit. Consequently, Scotland's net membership costs "could be as high as £500m"; it might even have to contribute to the remaining UK rebate.

"Third," he will say, "while the SNP have a working party on the 'affordability' of pensions, Scotland receives proportionately more spending on pensioners than the rest of the UK and more in incapacity benefit. With the rising number of pensioners in future years, Scotland will receive an even greater dividend from its membership of the United Kingdom."

Stewart Hosie, the Nationalists' Treasury spokesman, claimed the former PM's speech "unwittingly makes the case for a Yes vote in September because he had over a decade as Chancellor and Prime Minister to deliver these new powers for the Scottish Parliament and did none of them".

He added: "Even worse, as Prime Minister Mr Brown actually blocked transferring control of Air Passenger Duty and other powers to the Scottish Parliament even though this was ­specifically recommended by the Calman Commission."