And speaking at the same event as Mr Freeman, ERG chairman Jacob Rees-Mogg reiterated his support for Mrs May, telling the audience his party should "hang on" to the PM.
Supporters of Brexit acknowledge there may be some short-term pain for the UK's $2.9 trillion economy but say it will prosper in the long term when cut free from the EU which they cast as a failing German-dominated experiment in European integration.
It came as the EU's chief Brexit negotiator Michel Barnier suggested it was "realistic" to believe a Brexit deal could be agreed between the United Kingdom and European Union in the next six to eight weeks.
She predicted a leadership challenge when the Brexit deal was done.More news: Apple Bows to the Chinese Market Again With Latest iPhones
Former Brexit minister Steve Baker, who also resigned over Chequers and warned earlier this week that up to 80 Tory MPs could vote against it, said they were trying to "stay off" the leadership issue.
"Of all the years that I've sat in this chair, this coming year is one of the most hard to give guidance on because there is this uncertainty of the Brexit outcome", Associated British Foods finance director John Bason told Reuters.
She is aiming to strike a deal with Brussels by autumn, that will then be voted on by MPs in Britain, and the EU27.
He told Newsnight: "If you listen carefully to what Michel Barnier said there was no movement, there was an expression of confidence that we can get an agreement on the withdrawal agreement where we are going to give them £39 billion".
May's spokesman said her proposal, known as "Chequers" for the country house where it was hashed out in July, is the only serious, credible and negotiable plan available. Under Conservative rules, a leadership election is triggered if 15 percent of Conservative lawmakers, now 48 out of its 315 members of parliament (MPs), demand a vote of no confidence.More news: Weather Channel uses mixed reality to show the effects of a hurricane
The group argued that reverting to World Trade Organization rules, which can lead to the imposition of tariffs on certain goods, "would boost the U.K.'s trade with the rest of the world including Europe, lower domestic prices and boost inward investment".
"Now is the time for this Conservative government to show how a post-Brexit Britain will be a happy and dynamic economy that fosters enterprise, that rewards the strivers and the innovators, and where people can hope to take home more of their pay to their families", he wrote. "This is loose talk, you always have loose talk", Mr Gove told the 's Today programme.
The former deputy prime minister, who has been an outspoken critic of Johnson, told BBC Radio 4's The Week In Westminster: "Has he done himself any irreparable harm?"
"Any diversion or any distraction from that mission means that our ability to ensure that the referendum mandate that we were given is delivered, is undermined".More news: Astronaut snaps picture of Hurricane Florence as it makes landfall