Theresa May is due in Brussels on Thursday, hours after Conservative rebels in the Commons defeated the government in a key Brexit vote.
MPs backed an amendment giving them a legal guarantee of a vote on the final Brexit deal struck with Brussels.
One rebel, Stephen Hammond, was sacked by the prime minister as party vice chairman in the aftermath of the vote.
Other EU member states could decide to move forward to trade talks with the UK at their two-day summit.
The negotiations are first expected to focus on agreeing a temporary arrangement that will kick in as soon as the UK leaves the EU in March 2019.
On the eve of the summit, Mrs May suffered her first Commons defeat as prime minister by just four votes, as MPs backed an amendment to the EU Withdrawal Bill by 309 to 305.
Labour leader Jeremy Corbyn said it was a "humiliating loss of authority" for the prime minister.
Unless it is overturned by the government at a later stage, it means MPs will get to vote on the final deal reached with Brussels before it is ratified.
The EU Withdrawal Bill is a key part of the government's exit strategy.
MPs have been making hundreds of attempts to change its wording - but this is the first time one has succeeded.
Unless the government manages to overturn it further down the line, it means a new Act of Parliament will have to be passed before ministers can implement the withdrawal deal struck with Brussels, reports BBC.