(CNN) - The British government has the power to unilaterally halt the Brexit process, a top European legal adviser has said, delivering a significant boost to campaigners who want the UK to remain in the EU.
In an opinion prepared for the European Court of Justice (ECJ) in Strasbourg, the advocate general said the UK could stop the two-year legal countdown invoked under Article 50.
The UK had argued that the Article 50 notification could only be withdrawn with the agreement of all 27 remaining EU member states.
Announcing his conclusion Tuesday, Advocate General Campos Sanchez-Bordona declared that Article 50 "allows the unilateral revocation of the notification of the intention to withdraw from the EU."
Judges must now decide whether to accept the advocate general's advice, as they do in most cases.
If they do, it gives the UK parliament another way in which to force the government's hand. With only 16 weeks to go before the Article 50 deadline on March 29, options are running out if parliament rejects Prime Minister Theresa May's Brexit deal.
The advocate general's opinion comes in a case originally brought in Scotland by a cross-party group of anti-Brexit campaigners, led by Andy Wightman, a Scottish Green Party lawmaker in the Scottish Parliament. Scotland's top court, the Court of Session in Edinburgh, had referred it to Strasbourg for a ruling.
The case was fiercely opposed by the British government, which unsuccessfully argued that the Supreme Court in London should intervene before it went to Europe.
May told her Cabinet Tuesday that Brexit would not be reversed, a spokesman said. May stressed that the advocate general's findings were not binding on the court.
Jolyon Maugham, one of the petitioners in the case, welcomed the advocate general's opinion, urging UK lawmakers to "search their consciences and act in the best interests of the country."
"The decision is one that the UK can make unilaterally -- without needing the consent of the other Member States. That puts the decision about our future back into the hands of our own elected representatives -- where it belongs," he wrote on Twitter.
Conservative lawmaker Dominic Grieve, a former attorney general and second referendum supporter, told the BBC the decision by the advocate general was "clearly significant."
"Of course it doesn't necessarily have to be translated into a judgment, but the opinion of the advocate general is often very influential in forming the opinion of the court and it reinforces something I have to say I personally always thought was probably the case."
Asked whether the decision could lead to a second referendum on EU membership, he added: "It is certainly helpful because it removes one of the arguments which is 'Oh well, they would never allow us to change our minds'."
In his opinion, Sanchez-Bordona said the possibility of the UK unilaterally revoking Article 50 would be subject to "certain conditions and limits." He noted that it must "respect national constitutional requirements," and that would likely mean a vote in the UK parliament.
ECJ justices are expected to decide whether to accept the legal advice in the coming weeks. After that, the case will be referred back to the Court of Session in Edinburgh for a final ruling.
The decision came on the day that Theresa May begins her quest to sell the Brexit deal to lawmakers in the House of Commons.
MPs will spend the next five days debating the deal before a vote next Tuesday, which May is widely expected to lose.
Before Tuesday's debate begins, May could face another setback, when MPs vote on whether her government should be held in contempt of parliament for failing to publish the full legal advice on her deal.
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