Scotland's devolved parliament suspended a planned vote on Wednesday to give its government a mandate to seek a new independence referendum after an attack on Britain's Houses of Parliament in London which police said they were treating as a terrorist incident.

No date for the debate to resume was given.

The Scottish parliament issued a statement saying it would increase security measures, although no specific threat to Scotland had been detected.

London's permission for a new Scottish referendum is needed because any legally binding vote on United Kingdom constitutional matters has to be authorised by the UK parliament.

Prime Minister Theresa May has not completely ruled out another Scottish independence vote but has vowed to fight for what she has called the "precious union" of the United Kingdom.

First Minister Nicola Sturgeon, who argues that Scotland's vote to keep its EU membership in last June's referendum has been ignored in May's Brexit arrangements so far, is seeking authority for a second referendum from the Scottish parliament, to be held in late 2018 or early 2019.