The Labour ex-Prime Minister said devolved nations should be able to have their own relationship with the EU distinct from the rest of the UK in areas under their control.
And Mr Brown said MPs should be able to amend the final Brexit deal when it comes before the Commons, forcing the Prime Minister to go back to the negotiating table if her agreement isn’t good enough.
He also put pressure on the current Labour leader, backing the idea of a second EU referendum despite Jeremy Corbyn saying at the weekend that Brexit could not be reversed.
“We’ve got to insist that in an open democracy with an open process of democratic decision making… there is nothing wrong with saying the government has got to go back and renegotiate,” Mr Brown said in a speech at the Institute for Government in London.
“I dealt with the European Union for many years and the Irish used to come back and forth renegotiating on all sorts of things.
“You can’t say that there’s a fixed process - that once you’ve had one European Council [summit], it’s over. And of course, you've got the option of extending the Article 50 process.
I don’t think we should get into this mindset that the only choice is between the deal that’s offered - if there is a deal - and no deal. I think there are other options,and it's the duty of parliament to explore these options.”
He said there must be a proper debate and vote on any Brexit deal brought back to the Commons and it would be a "travesty of democracy" if that did not happen.
"I for one have always said that I think there will be a second referendum, I believe that in the end the situation will have been seen to have changed since 2016 and the people should in the end have the final say," Mr Brown said.
SNP politicians have complained of a 'power grab' over UK Government plans to share control of two-dozen devolved responsibilities currently administered in Edinburgh under EU law. The former Prime Minister suggested a royal commission to unite the two sides of the EU debate and the four nations of the UK.
Mr Brown claimed the “hard brexiteers of the Conservative Party” were “the English nationalist tail wagging the British bull dog”.
“The devolution [of powers after Brexit] that should have been automatic raises the question of how serious this government is about devolution - and if they don’t believe in it then people will start to draw conclusions,” he said.
“There is more centralisation that is going to take place in Whitehall as a result of leaving the European Union, because all of these powers are going to go back to Whitehall.
“It is a far more centralised britain that you will have after March next year.
“If these powers are not devolved then they’re held in the centre and the principle of devolution seems to be at risk.”
Recalling the arguments in the 2014 Scottish independence referendum, he added: “Nobody has come up with a way of allowing the question to be solved: if you promise that you’ll have access through the United Kingdom to the international community, and then you cut yourself from the European Union against the will of the Scottish people… what do you then do about it?
“It raises fundamental questions about the future of the Union, that are a risk to the Union.”