Advocate General Manuel Campos Sanchez-Bordona told the European Court of Justice that a British decision to revoke the countdown to departure would be legally valid.
European Union countries can unilaterally end the divorce process from the bloc, the legal advisor to its top court said Tuesday in a closely watched case launched by anti-Brexit politicians in Britain.
Clearly not trusting the rest of the European Union to agree with Britain if Parliament voted to reverse Brexit, the group petitioned the ECJ for the unilateral right to declare the United Kingdom under the continuing control of the exact same European Union that could not be counted upon to serve their interests in the first place.
The opinion comes after the UK's failure to kill the case and prevent it reaching the European Union court.
Two attempts by the United Kingdom government to appeal against the referral to the European court were rejected, and the case was opposed by the government and the EU institutions in a hearing before all 27 ECJ judges last week.
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 UK should be able to cancel its withdrawal from the EU without the consent of the other member states, a top European law officer has said.
She faces particular challenges from the pro-Brexit faction in her own party, which opposes several aspects of the agreement, including the Northern Ireland backstop, the goal of which is to avoid a hard border on the island of Ireland.More news: Sri Lankan court bars Rajapaksa from acting as Prime Minister
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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".
Ms Cherry tweeted that the opinion is a "massive vindication" for those who brought the case, adding: "We are optimistic the full court will follow this ruling".
Hubert Legal, the chief lawyer for the European Council, argued that allowing unilateral withdrawal could create "endless uncertainty" by allowing countries to announce they are leaving the EU in an attempt to secure better membership terms, before cancelling their withdrawal.
"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".
The "ECJ says the United Kingdom can revoke Article 50 without permission", Nigel Farage, the man who spearheaded the Brexit campaign, said on Twitter.
Legal representatives for the UK Government believe the case is inadmissible as it deals with a hypothetical situation, since the Government's policy is not to revoke Article 50.
Michael Gardner, a partner at law firm Wedlake Bell, said: "This will come as a huge boost to Remain campaigners who wish to stop Brexit". In that scenario, should the government seek in future to remain in the European Union, it could lose the perks of membership it now enjoys. They hope that if the court rules in their favour, it would pave the way for a potential second referendum, giving voters the option to remain in the EU.