Advocate General Manuel Campos Sanchez-Bordona told the European Court of Justice that a decision by the British government to change its mind about invoking the countdown to departure would be legally valid.
A top official at the European Union's highest court advised Tuesday that Britain can unilaterally change its mind about leaving the European Union, boosting hopes among to pro-EU campaigners in the United Kingdom that Brexit can be stopped.
The advocate general's opinion follows the news that MPs had submitted an emergency motion of contempt in response to the government's refusal to publish the full Brexit legal advice.
The opinion, which is not binding on the court, comes just a week after the case was heard at the ECJ following a referral from Scotland's highest civil court, the Court of Session.
The case to decide whether an European Union member state such as the United Kingdom can decide on its own to revoke the Article 50 withdrawal process, or whether the agreement of the 27 other member states would be required, was brought by a cross-party group of Scottish politicians.
A group of Scottish politicians has asked the court whether the United Kingdom can call off Brexit without the consent of other member states.
Welcoming the advocate general's opinion, SNP MEP Alyn Smith, one of those who brought the case, said it showed that "we now have a roadmap out of the Brexit shambles".
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 ECJ statement said the advocate general had proposed that the Court of Justice should "declare that Article 50 allows the unilateral revocation of the notification of the intention to withdraw from the EU".
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A final ruling on the case is due to be made at a later date.
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. In such a scenario, a renegotiation of the existing deal, general election, second referendum and no-deal Brexit could all be possible outcomes.
He also rejected the United Kingdom government's position that the case is purely hypothetical and therefore inadmissible.
European Union chief Brexit negotiator Michel Barnier.
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".
At the November 27 hearing before the full European Union court, lawyers for the lawmakers behind the case argued that legal certainty needs to come before politics and that the UK Parliament, which has the power to decide, needs to have certainty about the law.
"Plainly it is better for the country if we can unilaterally revoke the Article 50 notice", Maugham added.
Those who brought the case argue unilateral revocation is possible and believe it could pave the way for an alternative option to Brexit, such as a People's Vote to enable remaining in the EU.