The non-binding opinion was delivered by an advocate general of the European Court of Justice.
A judgment by the court will take place at a later date.
Five days of debate on the withdrawal deal - ahead of the December 11 vote - were set to commence on Tuesday afternoon, with an opening by the Prime Minister, though the start was delayed by the debate on the contempt of Parliament issue.
The legal opinion, drafted by the CJEU's advocate general, Manuel Campos Sanchez-Bordona, argues that the UK's withdrawal "may be revoked at any time" during the negotiating period, provided it is done in good faith.
According to British barrister Jolyon Maugham, one of the petitioners in the "Wightman case", this is a crucial point as if this was not the case then any cancellation (such as following a "People's Vote" or a general election or a decision by British MPs) could depend on the agreement of the other European Union leaders who might decide to set conditions such as the United Kingdom giving up various opt-outs.
The case comes as pressure builds from Brexit opponents for a second referendum on the decision to leave the EU.
In an opinion statement to judges at the European Court of Justice, published on Tuesday, Advocate General Campos Sanchez-Bordona pointed to a "third way" that was now open to the United Kingdom, which is facing the real prospect of a no-deal Brexit amid massive parliamentary opposition to the withdrawal agreement reached between the British government and the EU-27 group of nations in November.
It added: "That possibility continues to exist until such time as the withdrawal agreement is formally concluded".
National Hockey League unanimously votes to expand to Seattle, mayor says
Many expect the Arizona Coyotes to be moved to the Central Division in 2021 when Seattle enters the league. The committee voted 9-0 to recommend and forward the bid to the league's full board of governors.
May is trying to sell an agreement on a "smooth and orderly" Brexit to a hostile House of Commons, arguing that their choice is to back a deal or face the economic calamity of crashing out of Europe without a plan.
Who has brought the case?
British government lawyers argued in an European Union court hearing last month that the case was inadmissible because the United Kingdom had no intention to revoke its decision.
"That may well be positive for the pound in the short term but could be poisonous for politics in the United Kingdom", they added.
Downing Street has rejected a legal opinion arguing that the United Kingdom should have the right to unilaterally revoke Article 50 and stop Brexit.
"We know that if the Advocate General's opinion is followed we will know that we can keep all of the options and the special privileges that we enjoy as a member of the EU".
Noting the Vienna Convention on the Law of Treaties, the legal opinion states "notifications of withdrawal from an global treaty may be revoked at any time before they take effect".
It would nearly certainly need to be done by an act of parliament.
Although petitioners, including a group of politicians and lawyer Jolyon Maugham QC, want the United Kingdom to remain subject to European Union control, they appealed for absolute freedom from the bloc for the UK to make one key decision alone - to stay in the European Union.