"We want that to be as smooth and orderly a process as possible. It is not just in our interests, it is in the interests of Europe as well," May said as she arrived for a summit in Brussels ahead of the dinner.
May, who has promised to to trigger the two-year divorce process by the end of March 2017, said the fact that the other 27 European Union leaders would discuss their Brexit strategy without her was not a problem.
"I welcome the fact that the other leaders will be meeting to discuss Brexit tonight," she said, dismissing any suggestion that it amounted to a snub.
"We are going to invoke Article 50, trigger the negotiations by the end of March next year and it is right that the other leaders prepare for those negotiations as we have been preparing," she said.
All 28 EU leaders, including May, meet first to discuss a series of issues from how to deal with Russia over Ukraine and Syria to fraught relations with Turkey.
May then leaves and the other 27 to go into dinner, holding to their line that there can be no Brexit discussions of any description until she formally starts the negotiations.