"The Joint Committee may, before 1 July 2020, adopt a single decision extending the transition period for up to one or two years," Article 132 of the deal read.
It was specified that the European Union would notify "other parties to international agreements" if this decision was made.
READ MORE: LIVE UPDATES: Summit on Brexit Withdrawal Agreement in Brussels
Earlier in the day, the EU leaders held a special meeting on Brexit in Brussels. It took them less than an hour to formally endorse the Brexit withdrawal agreement and the political declaration regulating post-Brexit relations with the United Kingdom.
The text of the agreement is subject to final legal revision in the coming days, the European Council said on Sunday.
The United Kingdom is set to leave the European Union by late March 2019. The transition period is currently slated to last until December 31, 2020.