Jonathan Powell, who served as Downing Street chief of staff to Tony Blair, said the Prime Minister was "perfectly within his rights" in legal terms to reject another plebiscite on the constitution, despite the SNP's insistence it has a mandate to call one.
Mr Powell was the only senior adviser to last the whole of the Blair premiership and served as the chief British negotiator on Northern Ireland ahead of the Good Friday Agreement.
He described Brexit as an "existential threat" to the future of the UK which could lead to rising support for both a united Ireland and an independent Scotland.
"The British government promised in the Good Friday Agreement in 1998 to hold a border poll when a majority is in favour of leaving the UK. In Scotland, Johnson has said he will prevent a further referendum on independence even though support for leaving approaches 50 per cent.
"He is perfectly within his rights to do so in legal terms. But the consequence of denying self-determination to the people of Scotland will be to drive support for independence still higher, resulting in the same mess as Spain has with Catalonia.
"It is in any case untenable to agree to a referendum in Ireland, as we are required to by an international agreement, but to refuse one in Scotland. After all, British rule in both places depends on the consent of the people who live there."
Mr Powell's remarks come the day after Nicola Sturgeon laid out her “clear democratic case” for a second independence referendum. The First Minister said last week’s election victory in which her party took 47 of the 59 Scottish seats at Westminster, made the case for another referendum “unarguable”.
However in the Commons yesterday, Boris Johnson said that division had “poisoned” public life, and ruled out giving permission for another referendum.