British Prime Minister Theresa May is due Thursday to appeal to her EU counterparts for assurances that could help her sell the Brexit deal to the British parliament, with time running short to regulate Britain's departure from the European Union.

May's trip to Brussels, where EU leaders are holding their regular end-of-year summit, comes in a tumultuous week that has seen her delay a pivotal Brexit vote to avoid defeat, before standing her ground in a leadership challenge.

May said Wednesday she would ask the EU for "legal and political assurances" to sway the many lawmakers intent on voting against the Brexit deal because they fear it will surrender too much sovereignty to Brussels.

EU officials have insisted, however, that the agreement struck with London and endorsed by EU leaders last month is not up for renegotiation.

"Whatever reassurances will be given, they cannot contradict the deal which was agreed on November 25," a senior EU official said Wednesday on condition of anonymity.

At stake is the withdrawal agreement, which details the terms of Britain's departure from the EU on March 29, 2019, and a political declaration outlining London and Brussels' ambitions for the future relationship.

A key sticking point for eurosceptic parliamentarians is the so-called backstop, aimed at ensuring that no hard border will emerge between the Republic of Ireland and Northern Ireland, which will leave the EU with Britain.

The concern is that this would lock Britain into a close relationship with the EU that London cannot end unilaterally, while also treating Northern Ireland slightly differently from the rest of the United Kingdom.