UK Prime Minister Theresa May will formally invoke Article 50 on Wednesday 29 March, beginning the process of the UK's departure from the European Union following last June's referendum.
The move comes nine months after the UK voted 51.9% against 48.1% to leave the EU, and begins a two-year process of negotiations. Under Article 50, talks around the UK's departure cannot begin until the country has formally alerted the EU to its departure.
According to the Financial Times, the UK's EU ambassador Tim Barrow informed the office of the EU Council president on Monday morning that the formal notification would be delivered on 29 March.
However, Downing Street reportedly declined to comment on whether the letter would contain any details of negotiations, although May is expected to make a Commons statement on the same day.
The pre-announcement of the timing of the letter is partly aimed at preparing financial markets for the Brexit process, given the weakness and volatility of sterling since June's vote.
'Hard work begins'
Michael Stanes, investment director at Heartwood Investment Management, said: "The hard work now begins for the UK as it starts negotiations to exit the European Union. Triggering Article 50 no doubt marks a period of ongoing uncertainty for UK business and markets, but perhaps there is also some relief that the process is finally underway.
"While Brexit dominates UK concerns, French and German politicians will probably be more focused on their own national elections, which will further test anti-establishment sentiment. We continue to remain cautious on UK assets and expect higher inflation to weigh on real income growth this year."
