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 fears that the bill could constrain Holyrood’s powers in some key areas for up to seven years – although the vast majority of powers returning from Brussels after Brexit will go straight to the Scottish Parliament.
A protest against what the SNP claims is a “Westminster power grab” was due to be held outside the Scottish Parliament ahead of Thursday’s session of First Minister’s Questions.
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.
Mr Mundell will make a statement to MPs on Brexit and devolution later on Thursday, with the SNP also hopeful of securing a three-hour emergency Commons debate on the issue – which would potentially be held next week.
Speaking to the BBC’s Good Morning Scotland programme, Mr Blackford said his party at Westminster and 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.”
Mr Mundell, the Scottish secretary, is due to give a statement on the row to MPs at about 12:30 on Thursday.
Ahead of the statement, Mr Mundell said an agreement could still be reached “even at this late hour”, with the Brexit bill having completed its passage through the Commons.
He added: “Our commitment is that we will always seek consent in relation to the frameworks that will come forward as a result of this bill.
“So we won’t be power grabbing, we won’t be overriding the Scottish Parliament, we will be seeking their consent on all of the matters that this bill relates to”.