The Bank of England's financial regulator has warned it may have to take its eye off other areas as it faces a "material extra burden" from Brexit planning.
Sam Woods, chief executive of the Prudential Regulation Authority (PRA), said while the watchdog was having to make work on Brexit its "top priority", this would place a significant strain on its resources.
In a response to a letter from the new Treasury Select Committee chair Nicky Morgan, he said the PRA would likely have to make some tough decisions.
He said the prospect of authorising and supervising more firms set up as a result of contingency planning was "likely to place a material extra burden on the PRA's resources".
"It is incumbent on us to manage this burden but we may have to make some difficult prioritisation decisions in order to accommodate it," he added.
The warning comes after Ms Morgan last month called on Mr Woods to reveal how City firms are preparing for a cliff-edge Brexit and whether those plans threaten financial stability.
In his reply dated August 2, Mr Woods said the threat of fallout across the City from the UK's withdrawal from the EU poses a "material risk to our objectives and this work is therefore a top priority".
The PRA has collected details from more than 400 banks and financial firms with cross-border activities between the UK and the EU, including subsidiaries of US investment banks based in London.