While Advocate General Manuel Campos Sanchez-Bordona's opinion is purely advisory, the Luxembourg-based court usually follows such advice.
The ECJ has not yet made its ruling on the matter, but the breakthrough for pro-European Union activists in the United Kingdom came after the European advocate general gave his legal opinion on the matter.
Still, a decision saying that the so-called Article 50 notice can be unilaterally revoked favors those who want to remain in the European Union and could help those campaigning to thwart Brexit with a second referendum.
The issue is so complicated because while Article 50 of the EU's Lisbon Treaty tells member states how to start the process of leaving the bloc, it offers no help on what to do it they change their mind.
The news comes as the United Kingdom prime minister, Theresa May, gears up for a week of debates to try and pass her withdrawal agreement in the House of Commons - which is being seen as an uphill task.
The opinion comes after the UK's failure to kill the case and prevent it reaching the European Union court. In such a scenario, a renegotiation of the existing deal, general election, second referendum and no-deal Brexit could all be possible outcomes. An expedited ECJ decision on the case is expected before the end of the year.
The advice came as Prime Minister Theresa May pressed ahead with plans for a parliamentary debate on her divorce deal with the EU.
To allow unilateral revocation would open a "Pandora's box", they said, in which member states feigned to leave in order to get opt-outs or other perks in the exit talks, before making a U-turn.
The debate comes as the European Court of Justice announced on Tuesday that the United Kingdom can unilaterally revoke its withdrawal from the EU.
The case was brought forward in February by a group of Scottish politicians - Labour MEPs Catherine Stihler and David Martin, Joanna Cherry MP and Alyn Smith MEP of the SNP, and Green MSPs Andy Wightman and Ross Greer, together with lawyer Jolyon Maugham QC, director of the Good Law Project.
Who has brought the case?
He also rejected the United Kingdom government's position that the case is purely hypothetical and therefore inadmissible.
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".
Lawyers for the EU Council and the European Commission also argued that Article 50 could only be reversed by a unanimous decision of all 28 EU capitals.
"Plainly it is better for the country if we can unilaterally revoke the Article 50 notice", Maugham added.
If it was done by ministers alone using prerogative powers it would frustrate the will of parliament as expressed in the European Union (Withdrawal) Act 2018. Whereas extension of the Article 50 period could become a political necessity, revocation of Article 50 remains something of an academic point at present.
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.