Theresa May says she has “been clear” with the EU that she is only seeking a short delay to Brexit.
Mrs May wants to postpone the UK’s exit date beyond this Friday until 30 June – but EU leaders are expected to offer a longer delay, with conditions.
Arriving at a summit in Brussels, the PM said she “greatly regrets” that the UK has not already left.
Mrs May will meet the other 27 EU leaders at 17:00 BST to make her request for an extension.
The PM will then leave the meeting and the leaders will discuss her proposal over dinner.
If the EU agrees to the UK leaving when a deal is ratified by Parliament, the UK could still exit by 22 May.