Boris Johnson writes in the letter that he will not "negotiate a new deadline for when the Brexit should be implemented" while "extending the negotiations is not an option", reports Sky News.
Johnson warns MEPs that the EU may "deny Parliament's request for an extension of the deadline, or fail to make a decision immediately".
"In such a situation, I hope that all the members on both sides of Parliament - who are in a situation where there is between a withdrawal with an agreement or one without - choose the alternative with the new agreement".
"EU will approve extension"However, information to The Guardian claims that the EU will approve an application to extend Brexit. However, the EU will not take any initial initiatives for new negotiations.
Leaders of various EU countries should already have agreed at a meeting on Thursday that a future request would be approved. According to sources, French President Emanuel Macron should not have even been postponed.
A colleague of German Chancellor Angela Merkel also says that an application for extended Brexit negotiations would "without doubt" be approved by the EU.
On Saturday, EU Permanent President Donald Tusk tweeted that he is awaiting "a letter" from Boris Johnson after talking to him after the vote.
Ambassadors for the EU's 27 member countries will meet on Sunday to discuss Saturday's events.