Former deputy Labour leader Roy Hattersley has warned that Brexit would be "much worse" than staying in the the EU.
The veteran politician made the claim as he backed calls for another referendum.
Lord Hattersley said the public have a "right" to express a view again on whether they want to remain in the EU.
He added that "sometimes, you just have to do what is right" rather than what "wins elections".
The 86-year-old, who was a minister in the Wilson and Callaghan governments, was due to declare his support for the People’s Vote campaign for another referendum in a speech in Sheffield on Saturday, but he was forced to pull out due to illness.
But he told BBC Radio 4’s Today programme: "I think the British people have a right to cast a vote on the merits of the package Mrs May has negotiated.
"They voted by a small majority to leave the union, but they had no idea what leaving the union meant.
"We now know how bad it will be. We now know that it will be much worse than remaining in, and that the British people have a right to express a view on whether they want to remain in or they want to leave."
Around 500 people attended the Sheffield rally and sent their best wishes to Lord Hattersley with a round of applause.