Scotlands plans for a second independence referendum have wrong-footed Prime Minister Theresa May, who could now be forced into giving the go-ahead for the vote -- but only after Brexit.

Scottish First Minister Nicola Sturgeon's bombshell announcement on Monday came at the start of a week in which May had been expected to announce the start of the process for extracting Britain from the EU.

Downing Street later played down an imminent Brexit statement, which it said had always been planned for later in the month -- but British newspapers were left in no doubt as to what had happened.

"Sturgeon ambushes May," read a front-page headline in The Times, while the Guardian said: "May's Brexit plan upstaged as Sturgeon seizes her moment."

The Metro daily said: "Scots throw a sporran in the works" -- a reference to a distinctive leather pouch worn in traditional Scottish Highland dress.

Commentators pointed out the irony that May will now be forced to argue at the same time against membership of one union -- the European one -- but in favour of another -- the British one.

The European Commission said an independence referendum was a domestic matter for Britain but indicated that Scotland would have to reapply to the EU if it broke off from Britain.

Gianni Pittella, head of the socialist group in the European Parliament, told a briefing in Strasbourg on Tuesday: "The hard Brexit disaster is beginning to unfold with Scotland looking to get out of the UK."

May faces another headache in Northern Ireland.

Irish nationalists Sinn Fein made major gains in elections in the British-ruled province earlier this month and on Monday called for a referendum to leave Britain and unite with Ireland "as soon as possible".

"We're in unchartered waters," said Quentin Peel, associate fellow with the Europe programme at Chatham House, an international affairs think tank.

"We don't know which way it's going and we don't know if actually the Scottish threat combined with a possible threat of a Northern Ireland rebellion as well might actually at the end of the day stop Theresa May doing what she's doing," he said.

'Completely unacceptable' timing

May has the power to block a Scottish referendum but experts said she would be unwise to do so since this could push public opinion in favour of independence.

But she could object to Sturgeon's preferred timing of holding the vote by spring 2019, before Britain actually leaves the European Union.

Ruth Davidson, the leader of the Scottish Conservatives, said the timetable "would force people to vote blind" as they would not yet know the terms of the Brexit deal.

"The prime minister has said this would mean a vote while she was negotiating Brexit and I think that can be taken pretty clearly as a message that this timing is completely unacceptable," a government source was quoted as saying in The Times newspaper.

"It would be irresponsible to agree to it and we won't," the source said.

The Daily Telegraph also cited sources close to May saying she would not allow a referendum until "several months" after Britain has left the EU.

May on Monday said a Scottish independence vote would be "divisive and cause huge economic uncertainty" and would come at the "worst possible time".

She is expected to give a more detailed answer next week after Sturgeon receives formal authorisation from the semi-autonomous Scottish parliament to apply for permission to the British government.

Scotland voted against independence by 55 percent to 45 percent in a 2014 referendum but Sturgeon argues that circumstances have changed completely since last year's Brexit vote.

Scots backed staying in the EU by 62 percent to 38 percent in the June 23 referendum but the national vote was 52 percent in favour of Brexit.

The latest Scottish opinion polls indicate that support for remaining part of Britain is going down, although still showing a small majority.

"It's wide open,"Michael Keating, politics professor at the University of Aberdeen, told AFP.