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.
The advice of the advocate general is often, but not always, followed by the full court, whose final verdict is expected within weeks.
Scottish MPs and MEPs had brought the case on Article 50 - a 250-word clause inserted into the EU charter eight years ago, which had never been used until the United Kingdom voted to exit the European Union in the summer of 2016.
While the opinion to the EU's top court is not binding, the advice will give hope to Remainers who want to stop Britain leaving the bloc.
The debate, which is due to last for five days, will conclude with a December 11 vote in the Commons on the proposed withdrawal agreement.
The case comes as pressure builds from Brexit opponents for a second referendum on the decision to leave.
A court source told AFP that the decision could well be made before the end of the year, but the judges must first decide if the ECJ even accepts jurisdiction in the case.
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.
A Scottish court decided in September to seek the European Union tribunal's guidance in a case brought by Maugham, along with a group of Scottish and English lawmakers seeking to reverse the so-called Article 50 process.
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In his opinion, published on Tuesday, European Court of Justice advocate general Manuel Campos Sanchez-Bordona said the United Kingdom can "unilaterally" revoke its withdrawal from the EU. The UK government had appealed against the referral, saying the question was purely hypothetical as there is no prospect of the UK government or MPs asking to cancel Brexit.
Although the Advocate General's opinion is non-binding, the ECJ follow his opinions in the majority of cases.
Noting the Vienna Convention on the Law of Treaties, the legal opinion states "notifications of withdrawal from an worldwide treaty may be revoked at any time before they take effect".
Sanchez-Bordona on Tuesday rejected the arguments by the European Commission and EU governments, that revocation of Article 50 can only happen with the unanimous backing of the remaining 27 nations.
It would nearly certainly need to be done by an act of parliament.
"Plainly it is better for the country if we can unilaterally revoke the Article 50 notice", Maugham added.
Michel Barnier, the EU's chief Brexit negotiator, has previously said that the EU27 would need to provide their consent to reverse the Brexit process.
That period can be extended, but only with the agreement of all of the other 27 European Union states.
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.