She asked for an option for the UK to withdraw earlier if a deal is agreed, but admitted the delay means the UK may have to take part in the European parliament elections in late May.
The Prime Minister said the government is seeking to secure a deal that would allow the UK to leave the EU before the elections, but said it is making “responsible preparations” to take part if necessary.
“This impasse cannot be allowed to continue,” May wrote. “In the UK it is creating uncertainty and doing damage to faith in politics, while the European Union has a legitimate desire to move on to decisions about its own future.”
The deadline is currently set for 12 April, after which the UK would leave the EU without a deal if no agreement has been made.
The request comes amid reports European Council president Donald Tusk is preparing to offer the UK a 12-month “flexible” Brexit delay.
The plan, which will be put to EU leaders at a meeting next week, would allow the UK to withdraw earlier if a deal has been secured, the BBC reported, citing a senior EU source.