The European Union will have to terminate the UK's prolonged EU membership on 1 July if elections for British MEPs have not been held, leaked document said, according to the Financial Times.
According to the leaked document, the UK will have to take part in May votes if it wants an extension of more than three months.
"No extension should be granted beyond July 1 unless the European Parliament elections are held at the mandatory date," the paper said, as quoted by the Financial Times.
The UK Parliament will hold a number of important votes on the future of the Brexit process in the next few days, which will determine the nature of the future UK-EU relationship.
After repeated failed attempts to pass the government's Brexit deal, the UK parliament backed a motion seeking to extend the withdrawal period beyond 29 March on Thursday. The plan now needs to be agreed upon by 27 EU member states.