A British pro-European Union protestor at a rally in Rome
Theresa May, who decided not to attend the European Union’s 60th anniversary summit in Rome, is due to formally notify Brussels of Britain's intention to leave on Wednesday, kickstarting up to two years of discussions.
German Chancellor Angela Merkel said Britain would not be granted any concessions which undermined the free movement of goods, people, services and capital within the European single market.
She said: ”Some things are not for sale.”
French President Francois Hollande said every effort would be made to ensure the divorce was amicable but warned some pain was inevitable.
He said "It was Theresa May who chose not be here and it was the British who chose to take another path but we have to maintain good relations. France is very connected to the UK."
But Mr Hollande insisted the UK could not be seen to benefit from leaving the EU.
"We will ensure that Brexit does not happen to the detriment of Europe, that Britain remains a partner of the union but that, necessarily, it will pay the consequences."
EU officials suggested a more positive tone could have been set for the start of the negotiations had Mr May opted to travel to Rome for the largely ceremonial proceedings.
One senior official said: ”It is a shame she is not here.”
EU officials expect Britain's permanent representative in Brussels Tim Barrow to personally hand the withdrawal letter to European Council President Donald Tusk on Wednesday.
Draft guidelines for the remaining 27's negotiating stance are expected to be approved by ambassadors in Brussels on Friday before they are released, with Mr Tusk and Maltese Prime Minister Joseph Muscat due to discuss them at a press conference in Malta, on the same day.