LONDON — Scottish leader Nicola Sturgeon on Friday demanded that British Prime Minister Theresa May provide “urgent clarity” on plans for a transition period after Britain leaves the European Union, saying she is concerned the country is heading for a “no-deal” Brexit.

Britain is due to leave the EU in March 2019, but May suggested last month that the U.K. could keep paying the bloc in order to keep relations largely unchanged during a two-year “implementation period.”

This week, however, May suggested there could not be a transition agreement with the EU until there is a deal on future trade relations — a discussion that has yet to start, and could take years.

Britons voted in June 2016 to leave the EU, but most Scottish voters backed remaining in the 28-nation bloc.

Sturgeon, Scotland’s pro-independence first minister, has criticized May’s plans to take Britain out of the bloc’s tariff-free single market and its customs union.

In a letter to the prime minister released Friday, Sturgeon said May must offer “absolute clarity” that she is urgently seeking a transition agreement.

Sturgeon said was “increasingly concerned by the possibility that your negotiations may result in a ‘no deal’ scenario.”

She said the Scottish Parliament would try to stop that from happening because “no Brexit would be preferable to no deal.”

May’s office confirmed it had received the letter and would respond “in due course.”