Andrew Bailey, chief executive of the Financial Conduct Authority, speaks at his office in London, Britain, September 25, 2017. REUTERS/Afolabi Sotunde

LONDON (Reuters) - Financial firms in Britain will begin taking “irreversible” decisions about staff and operations by Christmas or early 2018 if there is no transition deal ahead of Brexit, Britain’s top markets watchdog said on Tuesday.

Andrew Bailey, chief executive of the Financial Conduct Authority said financial firms in Britain were already renting new buildings in the European Union, but such decisions could be reversed.

More “irreversible” contingency plans, namely putting staff in new operations in the EU to ensure continuity of operations after Britain leaves the bloc in 2019, are also more difficult to fulfil, Bailey told parliament’s Treasury Select Committee.

“That is why they and we tend to take the view that the end of this year, beginning of next year is the point at which these things start happening,” Bailey said.

A strong commitment by Britain and the EU to a transition deal after Brexit would ease pressure on firms to take irreversible decisions, he added.