Former Prime Minister Sir John Major has said it was “frankly absurd” for the SNP to claim Holyrood would not be given more powers in the event of a “no” vote.
He said the Conservatives, Labour and Liberal Democrats could not possibly turn their backs on commitments to keep the UK intact.
Sir John, who was in office between 1990-97, said he was “confident” that the Scottish Parliament would be strengthened in the future.
The three pro-union parties in Scotland teamed up on Monday to make a joint statement on a guarantee of more devolution in the event of a No vote.
They said they were committed to more taxation to give Holyrood more responsibility over the money it spends.
But a new opinion poll showed 43% of people asked said they did not trust Westminster on the issue.
SNP MSP Linda Fabiani said: “Past experience has shown that promises on further powers are quickly forgotten by the Westminster parties once they get what they want.
“It is clear the people of Scotland will not be fooled again”.
But Sir John, who succeeded Margaret Thatcher, dismissed the powers claim.
“It is frankly absurd to suggest they would simply be shelved after the referendum,” he added.
“They won’t be, they will be in-enacted.
“The three major political parties of the UK could not possibly resign from such explicit and important commitments.
Sir John said he was “confident” Holyrood would be strengthened.
He admitted he opposed devolution in 1992 because he feared it would weaken the union and would be a “stepping stone to independence”.
He explained he acted to try and save the union and was supporting more devolution today for the same reasons.
Sir John said: “I am fairly relaxed about the extent of devolution but what I am not remotely relaxed about is the break-up of the UK and ignoring the impact of devolution on the rest of the UK.”
He claimed Scotland, England, Wales and Northern Ireland would be “big losers” if the UK was fractured.
Sir John said voting “no” on September 18 meant “holding on to the advantages” of one of the most successful partnerships in world history.
He said he thought an independent Scotland would have trouble joining Nato and the European Union which he claimed could take years.
“Nations like Spain and Belgium with separatist tendencies in their own countries will not be keen to see any separatists waived easily into the European Union,” he added.
“Is it not bizarre that the SNP are campaigning to leave one highly successful union whilst applying to join another much less successful union that is seen to be in serious need of reform?
“It is even more difficult to understand when you consider there have been always Scottish voices around the cabinet table at Westminster and at the very top of our public services.
“By contrast, Scotland, who would be in population terms a mere 1% of the EU, would be guaranteed no senior positions in Europe whatsoever.”
Sir John said the SNP were campaigning to replace “real influence” in the UK with “possible irrelevance” in the EU.
He claimed the SNP threat to expel Trident nuclear warheads from Faslane was undermining the UK and Nato.
“America would not forgive nor forget and yet the separatists assume that membership of Nato is almost a given,” he added.
“Trust me ditch Trident it would be far from it.”
Sir John said it was unlikely that Alex Salmond would be given any special preference because he has “just wrecked the European military establishment” by getting rid of nuclear weapons.
He added that it would be “very difficult” for an independent Scotland to secure the same opt-outs and rebate that the UK currently enjoys.
“New nations coming into Europe are not getting favourable terms, they will have to join the queue and have to negotiate all sorts of things.”
Sir John said there was no evidence to support Scottish Government assertions that fishermen and farmers would get a better deal if Scotland was as member of the EU.
“If Scotland were to stay within the EU it would still be operating under the common agricultural policy as they are now so I am not at all sure I take their warning as credible.”
Sir John said it was time for First Minister Alex Salmond to “disown” people who verbally attack and abuse those who are opposed to independence.
Harry Potter author JK Rowling was subjected to vile online attacks after she donated £1million to the pro-union Better Together campaign group.
“If Alex Salmond cannot control these people, and perhaps he cant, then at least he should disown them and he should,” said the former Tory MP.
“He should say ‘we do not agree with these people, we disown you and we do not want you as part of our campaign’.
“The option is open to him to make his effort to stop this nastiness and then the SNP ought not to be blamed.
“It is an extreme group and he has to make it clear what they are doing is unacceptable and they should stop”.
Sir John said he thought a clear majority vote in favour of the union would “kill” the independence campaign for a generation and “push it back to be a minority cause”.
He added that Prime Minister David Cameron was right not to debate independence with Mr Salmond because it was too important an issue to be “boiled down into television sound bites”.
SNP MSP Stewart Maxwell said: “John Major is perfectly entitled to his view but his comments are woefully out of touch.
“He was wrong about a Scottish Parliament in the 1990s, and he is wrong about an independent Scotland now.
“He also, unwittingly, makes the case for independence himself by pointing out that the Tories rule Scotland with just one MP out of 59.
“But John Major is just about the last person the ‘no’ campaign will have wanted to see entering the debate here, given his track record in ensuring a Tory wipe-out in Scotland in 1997.”