Britain's triggering of Article 50 was "politically and constitutionally reckless" and denying Scots a second independence referendum before negotiations are over would be "democratically indefensible", Nicola Sturgeon says.
Scotland's first minister warned on Thursday that Brexit "threatens to be an act of self-harm on a scale barely understood" after Prime Minister Theresa May formally notified the European Union of Britain's intention to leave.
Writing in The Guardian, Sturgeon said the PM's proclamation in her letter the vote to leave was to "restore, as we see it, our national self-determination" meant the SNP has a "cast-iron democratic mandate" for a second vote on independence.
May has said she will block another referendum while the Brexit process takes place, saying: "Now is not the time."
Sturgeon wrote: "The Scottish parliament this week voted to mandate formal discussions with the UK government on the process to give the people of Scotland a choice on their future.
"For a prime minister who on Wednesday proclaimed Brexit as an exercise in self-determination to now seek to block Scotland's own right to self-determination would be democratically indefensible."
The SNP is campaigning for a second referendum to be held in about 18 months, allowing time for a Brexit deal to be struck and an independence vote held before the Article 50 period expires in March 2019.
Ms Sturgeon said leaving the bloc was expected to cost Scotland STG11 billion ($A18 billion) a year by 2030 and lead to 80,000 job losses during the next decade.
"The triggering of article 50 is also politically and constitutionally reckless," she said.
"The full effects on Northern Ireland, which currently faces the possible reintroduction of direct rule, remain to be seen.
"Similarly, there has been no serious attempt to engage with compromise proposals that would keep Scotland - which voted decisively to remain in Europe - inside the single market."
