An independent Scotland would be relegated to a "neocolonial" relationship with the remainder of the UK if it keeps the pound, according to former prime minister Gordon Brown.
He attacked the SNP's plan for a currency union, insisting it would cede control over "the important issues affecting our lives" to a Westminster government in which it would no longer have a say.
In an address to the Edinburgh Book Festival, he said "too much power rests in London" and insisted the pledges of further devolution if Scotland votes No will create a near federal UK.
He recognised that the "corruption" of the expenses scandal had dented people's trust in MPs but insisted unionist MPs have "no alternative" but to keep their devolution promises at the risk of a Scottish backlash in the general election.
On the SNP's plan to keep the pound, Mr Brown said: "On the important issues affecting our lives - interest rates, money supply, inflation, what happens to our living standards, employment, banking rescues - when it came to it we would have no formal input on the decisions being made.
"We would have to accept them being made elsewhere and we would be creating by our own decision, with the SNP, a neocolonial relationship between us and the rest of the UK where we would have to accept the decisions that were made.
"It is a worse outcome than now and it is the worst possible outcome for Scotland."
The Oxford Dictionary defines neocolonialism as the use of economic, political, cultural or other pressures to control or influence another country, especially the retention of such influence over a developing country by a former colonial power.
Mr Brown said the British constitution "is broken and has to reform".
"We're going to be, within a year or two, as close to a federal state as you can be in a country where one nation is 85% of the population," he said.
"There is now an all-party agreement that they will bring in big changes after the referendum.
"I believe that there is no alternative but to introduce these changes, which would be taxation powers for the parliament.
"It would mean that there would never be anything like a poll tax or a bedroom tax or rail privatisation imposed on Scotland against its will."
Mr Brown said "there is a mood that too much power rests in London" but that this is about to change.
"When I was at university I heard a politician speaking and I'll never forget what he said: 'This is a promise we will keep'," Mr Brown said.
"I can understand how people are sceptical when promises are made. I would say that there is no alternative to further devolution.
"It's not just that all parties have committed themselves to it, it's that the people of Scotland know that these promises have been made, not just by one party but all the parties that they want to see further change.
"These parties will not command popular support in the general election in Scotland if they renege on their promises after the referendum.
"I would suggest that the talks to agree this devolution should start immediately after September 18 so that we recognise that there is a will for change."
A spokesman for Scotland's Finance Secretary John Swinney said: "An independent Scotland will keep the pound, which is ours as much as the rest of the UK's.
"The problem for Gordon Brown and the rest of the No campaign is that they have no Plan A for more powers for Scotland in the event of a No vote - which is why more and more people are deciding to vote Yes on September 18."