Sir John Major has threatened leadership hopeful Boris Johnson with legal action if he were to suspend Parliament to force through a no-deal Brexit.
The former Prime Minister said it would be "utterly and totally unacceptable" for any British premier to shut down Parliament, and he would seek a judicial review if it happened.
"You cannot and should not bypass Parliament in this fashion," said Sir John, who prorogued Parliament himself in 1997.
"I cannot imagine how anyone could conceivably think that is right."
Mr Johnson has refused to rule out the idea of proroguing Parliament to prevent MPs blocking a no-deal exit from the European Union on October 31, and he reiterated his stance during a debate with leadership rival Jeremy Hunt last night.
"I'm not going to take anything off the table, any more than I'm going to take no-deal off the table," he said.
But Mr Hunt was vocal about his opposition to suspending Parliament.
He told the audience: "When that has happened in the past, when Parliament has been shut down against its will, we actually had a civil war."
In order to prorogue Parliament, shutting it down until the next state opening, a Prime Minister would have to ask the Queen to formally allow it.
Sir John, who is backing Mr Hunt in the leadership race, added: "I for one would be prepared to go and seek judicial review to prevent Parliament being bypassed."