Investment bank JPMorgan sees an "uncomfortably high" 25% chance that Britain's Brexit transition period will end in a 'no-deal' scenario when the deadline to strike a trade deal with the European Union ends in Dec. 2020.

Prime Minister Boris Johnson has sent sterling tumbling with his plan to cement the 2020 deadline in law, meaning he will have just 11 months to strike a trade deal. That has again raised the risk of exiting the bloc without trading arrangements in place.

"We have put the risk of a "no deal" end to the transition at 25%, a number we regard as uncomfortably high. The negotiation process is path dependent and we could find ourselves on that path even though neither negotiating views it as their first preference," JPMorgan said in a note received on Wednesday.

But it said a new treaty with a new trade relationship remained most likely, with a 50% probability, up from 30% previously. It halved the likelihood of the transition period being extended into 2021 to 20%.

(Reporting by Sujata Rao; editing by Mike Dolan) ((; +44 207 542 6176;))