Mr Barnier successfully pressed for the European Parliament’s formal negotiating stance to include a line clarifying that Britain cannot halt the process without the approval of the 27 member states.
It came amid concern that the UK may temporarily halt the two-year countdown to Brexit, started by the triggering of Article 50, in order to create more time to settle a deal.
The Commission has stated that EU treaties would not allow the UK to unilaterally halt Brexit, but the issue is as of yet untested in court and Lord Kerr, the British author of Article 50, has suggested the process is reversible.
Socialist group leader Gianni Pittella told reporters at a dinner in Strasbourg that Mr Barnier had asked for the wording on revocation. A spokesman later said MEPs had always been in full agreement with Mr Barnier on the issue.
Asked about Mr Barnier’s alleged request, a European commission source told The Guardian: “Article 50 does not provide for the unilateral withdrawal of notification. In any dealings with the European Parliament, Michel Barnier reiterated the above principle.”
The European Parliament resolution, passed last Wednesday, said: “A revocation of notification needs to be subject to conditions set by all EU27 [states] so they cannot be used as a procedural device or abused in an attempt to improve the actual terms of the United Kingdom’s membership.”
The motion is not legally binding, but the guidelines will feed into the overall mandate adopted by Mr Barnier and the institution has a veto over any future deal between the UK and the EU.
Campaigners, led by the barrister Jolyon Maugham, filed papers to the high court in Dublin earlier this week, asking for the question of Britain’s ability to act unilaterally to be referred to the European Court of Justice.
They want to empower the Prime Minister to revoke Article 50 in the event of the House of Commons rejecting a poor deal when it goes to a vote of MPs in 2019.
During debate on Article 50 in the House of Lords, Lord Kerr told peers: “During that period, if a country were to decide, ‘Actually we don’t want to leave after all,’ everybody would be very cross about it being a waste of time.
“They might try to extract a political price, but legally they couldn’t insist that you leave.”