The discussion, which will take place after the Prime Minister has left a summit meeting of all EU leaders, is expected to see the European Commission’s top Brexit official Michel Barnier given the lead role in the negotiations.
Arriving in Brussels for the regular EU gathering, Mrs May said: “I welcome the fact that the other leaders will be meeting to discuss Brexit tonight. As we are going to invoke Article 50, trigger the negotiations, by the end of March next year, it’s right that the other leaders prepare for those negotiations as we have been preparing.
“We will be leaving the EU, and we want that to be as smooth and an orderly process as possible. It’s not just in our interests, it’s in the interest of the rest of Europe as well.”
Mrs May ignored repeated questions about private warnings to the Government from Britain’s EU ambassador Sir Ivan Rogers that a post-Brexit trade deal could take a decade to finalise and even then may fail to get ratified by member states.
The PM said: “Well, what we will be discussing at this summit is how we work together to deal with the serious challenges that we face.
“So, we will be discussing migration. I have always said from the outset on migration that Europe needs to do more to tackle the root causes.
“That means disrupting the smuggling networks. It means deterring more people from taking the journey in the first place, and returning those who have no right to be here.
“We also need to robustly condemn what is happening in Syria. President Assad and his backers in Russia and Iran bear responsibility for the tragedy in Aleppo.
“What we must be doing is ensuring that those who are responsible for these atrocities are held to account. We must also do all we can to ensure that a ceasefire is secured so the United Nations can help to bring to safety the innocent people of Aleppo.
The discussion by the 27 other EU leaders without Britain, which is expected to take place over a working dinner in the Belgium capital, will decide how the EU will organise itself for the talks which will follow Mrs May triggering Article 50, the formal legal mechanism which officially begins the two-year countdown to Brexit.
But there could be complications as MEPs claimed the leaders’ plan - revealed in a leaked draft - sidelined the European Parliament’s Brexit negotiator Guy Verhofstadt.
The former Belgian prime minister said: “If the government leaders do not take the European Parliament’s role seriously, we’ll negotiate directly with the British. If that’s what they want, they’ll get it.”
Giving officials from president Jean-Claude Juncker’s European Commission the central role could also weaken the ability of Mrs May and her ministers to use their influence directly with counterparts in other governments to secure a favourable Brexit deal.
Mrs May will hold talks with the outgoing president of the European Parliament Martin Schulz in the margins of the Brussels summit.
She will also meet Latvian prime minister Maris Kucinskis and Lithuanian president Dalia Grybauskaite, meaning that she will have held talks with leaders from all EU countries apart from Austria and Bulgaria by the time the summit starts.