The SNP will continue to “frustrate what the government are doing as much as we possibly can”, the party’s Westminster leader has said.
Ian Blackford led a walk-out of his MPs during Prime Minister’s Questions on Wednesday over the government’s handling of the Brexit bill.
Opponents branded the move a “pre-prepared stunt” aimed only at furthering the cause of independence.
Mr Blackford said it was “not the end of the matter, it is the beginning”.
The walk-out came after Speaker John Bercow expelled Mr Blackford from the chamber when the MP refused to sit down after asking for the Commons to sit in private.
SNP MPs were furious after amendments to the EU Withdrawal Bill affecting Scotland were passed after less than 20 minutes of debate the previous evening, with the only speech being from Cabinet Office minister David Lidington.
Holyrood had previously voted against granting formal consent to the bill, with Labour, Green and Lib Dem MSPs uniting with the SNP to outvote the Conservatives.
The opposition centres on the bill “temporarily” constraining Holyrood’s ability to use some of the powers that return from Brussels for up to seven years.
The vast majority of powers that return will go straight to the Scottish Parliament, however.
A protest against what the SNP claims is a “Westminster power grab” was held outside the Scottish Parliament ahead of Thursday’s session of First Minister’s Questions.
Ms Sturgeon used FMQs to argue that this week’s events had been “the most clear and powerful evidence so far that the Westminster system simply does not work for Scotland”.
And she repeated her claim that the Tories had “ripped up the convention that has underpinned devolution for nigh on 20 years.”
The UK government has signalled that it wants to continue working with its opponents towards finding a resolution to the long-running row, with Scottish Secretary David Mundell saying it was still not too late to come to an agreement.
In a statement to MPs on Thursday afternoon, Mr Mundell said the powers at the centre of the row had been exercised by Brussels since 1972, and had therefore never been under the control of Holyrood.
He said the government would press on with its Brexit plans despite the lack of consent from the Scottish Parliament, but insisted that its aim was still for agreement to be reached between the two sides.
Speaking to the BBC’s Good Morning Scotland programme earlier on Thursday, Mr Blackford said his party at Westminster and the Scottish government in Edinburgh would mount a “very robust defence of our parliamentary democracy, our parliamentary sovereignty and the rights of the Scottish people”.
He insisted Wednesday’s walk-out, which was backed by First Minister Nicola Sturgeon, “certainly hadn’t been a stunt” and that the UK was now in the midst of a “constitutional crisis”.
And he warned that a failure by the UK government to “think again” on the Brexit bill and its impact on the devolution settlement would see the SNP “take whatever action is necessary”.
Mr Blackford added: “I will make sure that we can frustrate as much as we possibly can what the government are doing.
“We will remain civil, we will remain polite, we will remain courteous. But they need to understand that a line has now been crossed – the Conservatives are enacting legislation without the support of the Scottish Parliament.
“We are now in different territory.”
Mr Blackford hinted that his party would next target the government’s Trade Bill, which aims to ensure the UK can continue its existing trade policy as far as possible immediately after Brexit.
Scottish Conservative MP Andrew Bowie hit back at claims that the Withdrawal Bill was a Westminster “power grab”.
He said: “Due to the legislation we passed this week 120 extra powers will be going to Holyrood – this is going to enhance the devolution settlement, not in any way restrict it.”
He conceded that there “should have been a much more substantial debate” on Tuesday, but blamed Labour for this, saying most of the time had been taken up with votes in the Commons that had been pressed for by Jeremy Corbyn’s party.
Mr Bowie also hit out at Mr Blackford, saying: “By walking out of the chamber in a pre-prepared parliamentary stunt he actually gave up the opportunity to hold the government to account and actually have a debate about the very issue he is complaining he hasn’t had a debate on.”