Gordon Brown last night gave his strongest signal yet that Holyrood should have more tax-raising powers as he launched a withering attack on the SNP's "bleak separatist obsession".

Just hours after Charles Clarke, the former home secretary, told the Prime Minister to "improve or go", Mr Brown sought to get his autumn relaunch back on track by pledging that his government would do everything in its power to help preserving the United Kingdom was "not about partnership at the expense of pride nor about pride that can be satisfied only by sacrificing partnership".

The Prime Minister then quoted Chris Hoy, the triple-gold medal Olympic cyclist, saying: "Scotland is part of Britain, they are not mutually exclusive. I wouldn't have three gold medals hanging round my neck if I wasn't part of the British team."

In a separate development, the 32-year-old Edinburgh-born athlete yesterday voiced his dismay at attempts by politicians to "cash in" on his success at the Beijing games.

Commenting about the proposal by Alex Salmond to have a Scottish Olympic team, Mr Hoy said: "I was annoyed at getting drawn into a political debate when I'm an athlete; I ride a bike, I'm not a politician."

In his speech, the Prime Minister spoke of the "real risk" of one day waking up to find the many benefits of the Union had been "too long taken for granted and thoughtlessly thrown away".

He explained that in a globalised economy the "simple arithmetic of the Union" meant that Scotland now generated more money through trade with England than it did from its trade across the entire European Union.

"Set against the global challenges facing us today, the bleak separatist obsession of the Nationalists to split Scotland from the rest of the UK looks, at best, self-indulgent posturing, at worst, like a wilful denial of the realities of the world we live in."

Mr Brown also criticised the Nationalist Government's decision to rule out more nuclear power north of the border, its proposal for a local income tax as "a tax on jobs" and its "refusal to invest in infrastructure".

families through the global economic downturn and made clear he was "cautiously optimistic" about Britain's ability to ride out the financial storm.

However, as he stood up to address CBI Scotland's annual dinner at the Glasgow Hilton, it emerged that the prospect of fuel vouchers of up to £100 per head had been ruled out with Whitehall insisting the focus would now be on energy efficiency. The development will disappoint many Labour MPs, who had been calling for the UK Government to slap a windfall tax on the energy giants' "excessive" profits, to pay for fuel aid.

In his speech, Mr Brown sought to calm the fears sparked by Alistair Darling's remarks in a newspaper interview which helped to send the pound plummeting on the foreign exchanges.

"Undoubtedly we face a challenging period in the British economy, particularly given our position at the heart of the world's financial markets and both the Chancellor and I understand the difficulties you face," he said.

"But while never complacent about our economic prospects, I am also cautiously optimistic about the long-term resilience and underlying strengths of the British economy."

He held out the prospect of at least one million new jobs being created in the new "green economy" by 2030 if British companies were prepared to invest in low-carbon technologies.

"The policies we are putting in place will mean that by 2020 our economy will consume 20% less oil for each unit of output than it consumes today - and only a quarter of the oil we used in 1970."

While much of Mr Brown's speech was taken up by economic matters and his desire to free Britain from the "dictatorship of oil", its political focus was on Scotland and the future of the devolution settlement.

Once again, the PM staunchly defended the Union but in doing so stressed: "Do not confuse that resolve with unthinking opposition to change and development in how our Union governs itself."

While he claimed he was not going to prejudge the Calman Commission - examining the powers of the Scottish Parliament - he appeared to do just that and in doing so accepted the direction of travel of political thinking and public opinion in Scotland towards greater powers for Holyrood.

"First of all," he told Scottish business leaders, "devolution has worked but I do see one problem. While there have been good reasons why this is so, the Scottish Parliament is wholly accountable for the budget it spends but not for the size of its budget. And that budget is not linked to the success of the Scottish economy. That is why we asked the commission to look carefully at the financial accountability of the Scottish Parliament and this is a critical part of Calman's remit."

Mr Brown said while devolution was intended to preserve the Union, "developing devolution" was intended to strengthen Scotland's place within it. He insisted the modern case for