British Prime Minister Theresa May arrives at the Council of the European Union on the first day of the European Council leaders' summit on June 28, 2018 in Brussels, Belgium.

European countries are expecting a letter from Prime Minister Theresa May on Wednesday, requesting a delay to the country's departure from the bloc.

"The letter to President (Donald) Tusk will be sent ahead of the European Council," a U.K. official based in Brussels told CNBC Tuesday.

The letter from May is due to arrive in Brussels before the heads of state gather on Thursday for an important summit. The 27 leaders will then be discussing the implications of an extension to the Brexit process.

However, a senior EU official said Wednesday morning that the letter has not yet arrived in Brussels and that the U.K. has not requested an extension in any other form. "As it stands, there is no request," the official said. "Maybe it will come, maybe it will not come."

Any delay to the U.K.'s departure from the EU needs to originate in London first. It is up to Theresa May to tell the other 27 heads of state that she needs an extension and why.

European Commission President Jean-Claude Juncker said Wednesday morning that May "must bring clear plans to the EU summit if she wants an extension on Brexit" but said it is "highly probable" that Britain will not leave the EU on March 29. Speaking to Deutschlandfunk radio, he reiterated that there would be no re-negotiations of the Brexit deal.

"When it comes to Brexit we're in God's hands. But even God has a limit to his patience," he said.