Thousands of people have backed a Brexit legal challenge to find out if Article 50 is reversible
If successful, the case could see Parliament able to reverse Article 50 if Theresa May's government couldn't secure a deal to their satisfaction - raising the possibility of halting Brexit entirely.
The case is being led by Jolyon Maugham, a tax barrister, and it launched a crowdfunding campaign to raise funds on Friday.
By Sunday it has reached its £70,000 target, with donations from almost 2,000 people.
"If a notification under Article 50 can be revoked, voters will get to see whether what they were told was true or false. And if it proves false, and damaging to their economic security, it will be open to them to choose to change their minds," a message on its fundraising page said. "And there's absolutely nothing wrong with that."
The case is being brought in an Irish court, and the campaigners say they plan to ask the court to refer the case to the European Court of Justice, the European Union's top court. "One or more" Members of the European Union are apparently willing to act as claimants, but they have not yet been named.
The case will also try and get an answer as to whether triggering Article 50 will automatically remove Britain from the European Economic Area, or if a separate process will be required.
Once Article 50 is triggered by a European Union member state, then negotiations for its exit can officially begin, with a two-year deadline before it is automatically ejected by the 28-member bloc - with or without a finalised deal.
If Article 50 is reversible, then it gives the UK government significantly more power in negotiations, campaigners say - as it will allow it to reject a deal it considers unsuitable, withdraw Article 50, and remain a member of the European Union.
The challenge is a fresh headache for Theresa May's government, which is already facing at least two other legal challenges:
- One over the triggering of Article 50. The case, heard by the Supreme Court last week, is examining whether Parliamentary approval is required to trigger Article 50 and begin the process of leaving the European Union. The government insists no vote is required, but campaigners, led by Gina Miller, argue otherwise.
- And one over Britain's membership of the European Economic Area. The government argues that triggering Article 50 will also begin the process of Britain withdrawing from The EAA. But thinktank British Influence disputes this, arguing that only a vote from Parliament can legally withdraw the UK from the EAA.
So far the British government has provided little clarity about the terms of the Brexit deal it will push for ("Brexit means Brexit!"), arguing that doing so would weaken its negotiating position. However, facing opposition from Labour and Europhile Tory backbenchers, the government has now agreed to publish a "plan" for Brexit in advance of triggering Article 50 by March 31, 2016.