The former Prime Minister used a speech last night at the Edinburgh International Book Festival to issue a plea – the day after the London Olympics ended – to keep the Scottish and English qualities that he said had made Britain one of the most fair countries in the world.
He dismissed the suggestion that the SNP could keep the monarchy, sterling and Nato membership if the country goes it alone and "pretend this would somehow soften the blow".
The former Labour leader said: "If you break up the political union and the pooling and sharing involved in that, it's clear you will either have to cut public expenditure massively or tax people more. There is no escape from that."
He added: "My worry about fiscal autonomy, which is now being proposed as the next stage of devolution, is that fiscal autonomy means more taxes in Scotland, not in a progressive way at all but simply to fill the gap that's left by not pooling and sharing the resources of the UK."
Mr Brown was giving the annual National Library of Scotland lecture in honour of his late friend, Scotland's founding First Minister, Donald Dewar. He argued Scotland brought to the Union its own qualities of justice and community rooted in the democratic intellect.
When these were combined with English values of "ordered liberty and individualism", Mr Brown said it resulted in a United Kingdom that had become one of the world's most successful nations. From that, he argued, came the NHS, the welfare system, the BBC, the armed forces and the London Olympics. He highlighted the cycling successes and Danny Boyle's opening ceremony with its sense of "tolerance, liberty and fair play".
Referring to the "pooling and sharing" of risks and resources, he cited the crisis in Scottish banks RBS and HBOS in the same breath as Scots-born gold medal-winning Olympic cyclist Sir Chris Hoy, who represented Team GB.
He added: "Break up the pooling and sharing and we will not be able to do what we have done with the Olympics."
In assessing the UK Government's performance, he said: "To say that if we have austerity we should have more austerity, that's a mistake. Do you give the hope that the UK can never prosper again? They did that in the eighties."
He said an independent Scotland that kept the pound would be relegated to "a colonial relationship" with the rest of the UK, and that different wage levels and taxes would start "a race to the bottom with one unit trying to undercut the other".
Mr Brown, the MP for Kirkcaldy and Cowdenbeath, said: "The good will undercut the bad and the bad will undercut the worst."
A spokesman for Finance Secretary John Swinney said: "Gordon Brown's attack on fiscal autonomy can only
backfire on the anti-independence campaign because the more the Unionist parties continue to offer Scotland nothing, the more they will encourage people to vote Yes to an independent Scotland.
"Scotland is better off than the UK as a whole, and therefore Gordon Brown is wrong about tax and spending, and wrong about fiscal autonomy and independence."
SNP Westminster Treasury spokesman Stewart Hosie MP added that Mr Brown was right to talk about principles.
He said: "The very first principle is that it is fundamentally better for us all if decisions about Scotland's future are taken by the people who care most about Scotland, that is, by the people of Scotland.
"That is why we want an independent Scotland that stands alongside the other nations on these islands in a social union of equals."
Mr Hosie said the former Prime Minister had got his GDP figures wrong, and Scotland's was 15% higher than the UK's overall. He said that after independence, it would be the sixth-richest nation in the developed world in terms of GDP per head, compared to the UK's 16th place.