The British ambassador to the European Union arrived at the blocs headquarters Wednesday ahead of the formal handover of a historic letter announcing Britains intention to leave.

Tim Barrow, wearing a waistcoat and dark suit and carrying a leather briefcase, left Britain's EU embassy in Brussels and stepped into a dark Jaguar car, AFP reporters said.

Minutes later he arrived at the Europa building less than a quarter of a mile (300 metres) away where he is due to deliver by hand the letter to EU President Donald Tusk at around 1120 GMT.

"Good morning," the bearded diplomat, who took up the post in January after his predecessor Ivan Rogers resigned, said to waiting reporters.

Barrow will first take part in a scheduled meeting with ambassadors from the other 27 European Union nations before the meeting with Tusk.

"At 13:20 today, UK #Brexit notification letter (article 50) will be handed to me by Ambassador Tim Barrow," Tusk said.

British Prime Minister Theresa May signed the letter on Tuesday, a photo released by her office showed.

Officials declined to say how the letter was being conveyed to Brussels, citing security reasons.

May is due to formally announce that she has triggered the Brexit process to parliament at 1130 GMT.

"We are one great union of people and nations with a proud history and a bright future. And, now that the decision has been made to leave the EU, it is time to come together," she will say, according to the speech published in advance by Downing Street.