Mr Jack made the claim during a Commons clash with a string of SNP MPs over the Government’s UK Internal Market Bill and its impact on Scotland.
It comes as Boris Johnson’s Government continues last-ditch talks with the EU to strike a post-Brexit trade deal.
SNP MP Stephen Flynn (Aberdeen South) branded the Internal Market Bill “a blatant attack on devolution”.
He said: “Just three weeks ago, the Prime Minister said that devolution was Tony Blair’s biggest mistake, a bigger mistake than even the illegal Iraq war. So I ask the Secretary of State, does he disagree with the PM?”
Responding, Mr Jack, pictured above, said: “What the Prime Minister said was that devolution was a mistake when it was set up to be put in the hands of separatists, and I completely agree with that.
“The SNP is a campaigning organisation for independence, for separation of the UK, masquerading as a party of government.”
SNP MP Mhairi Black (Paisley and Renfrewshire South) called on Mr Jack to “now make the case for Scotland to get the same concessions and, like his predecessor, will he consider his position if no such request is granted?”
She asked: “Does he (Mr Jack) feel that the announcement yesterday from the Chancellor of the Duchy of Lancaster that Northern Ireland will have the best of both worlds, meaning Northern Ireland will still have access to both EU and UK markets after Brexit, does he feel that this has undermined his claims about the Internal Market Bill?”
Mr Jack replied: “Not remotely, what we’ve delivered is unfettered access. We promised unfettered access at the time of the Withdrawal Agreement and we’ve delivered it.”
SNP Hannah Bardell (Livingston) said: “The Secretary of State would do well to remember that the SNP is a democratically elected party of Government in Scotland.”
She referred to independence polls, adding: “Does the Secretary of State believe that his Government’s disastrous Internal Market Bill has contributed to this rise in support for the SNP and Scottish independence?”
Mr Jack insisted the Union was “absolutely firing on all cylinders”.
He replied: “There’s nothing disastrous about a UK Internal Market Bill that … protects jobs in Scotland, protects people’s livelihoods, when 60% of Scotland’s trade is to the rest of the UK, worth over £50 billion pounds.”
On Brexit transition, SNP Stewart Hosie (Dundee East) called on Mr Jack to “insist there is a minimum six-month grace period so that businesses do not fall foul of regulations which have not yet been developed for a deal that is not yet agreed but which is supposed to be in place in barely three weeks’ time”.
Mr Jack replied: “We’ve always been clear whether it’s deal or no deal, there are steps that have to be taken when the transition period comes to an end.
“We’re not going to delay the end of the transition period because it’s only by sticking to that date people can prepare responsibly and it also holds, if you like, the EU’s feet to the fire in getting a deal.”