His comments follow reports that some within the Cabinet are pushing Mrs May to back a Canada-style deal, if the EU continues to reject the Chequers proposals.
Since walking out of the Cabinet in protest at Mrs May’s Brexit plans, Mr Johnson and Mr Davis have both called for “Super Canada” deals.
Mr Grayling said: “Most of the Brexiteers are still in the Cabinet.
“David Davis and Boris Johnson chose not to stay but the rest of us have backed the proposal.”
But, he went on, it is now “for the EU to come back to us with their plans. “The lesson of the last week is, ‘OK you don’t like Chequers, what are you proposing?’ ”
He added: “My message to Sunday Express readers is, we have to do two things.
“We have to fulfil the result of the referendum… We have got to protect the union that is the UK and be really robust with the EU about not accepting the concept they want: a border between Northern Ireland and Great Britain.”
He added: “I believe the Chequers proposal is a credible way of doing that and we have heard nothing else credible from the other side.
“Saying that we are going to have the same product specifications as are in the rest of the EU is just what is going to happen, because companies are not going to make a different product for the UK and France.
“But does this take back control of our borders? Yes. Does it take back control of agriculture and fisheries? Yes.
“What the Chequers proposal does is allow the smooth flow of trade across borders and provides protection for the car manufacturers and others based in the UK, without compromising the principles of the referendum.”
He said that Mrs May could never accept a separate customs regime for Northern Ireland.
But he suggested that Jeremy Corbyn could sign up to such a proposal.
“The Labour leader is a dangerous politician who would break up the United Kingdom,” he said.
“He is someone who would countenance demilitarising the United Kingdom.”
Mr Grayling also called on the EU to match Mrs May’s pledge to guarantee the rights of all EU citizens who live in the UK even if we leave the bloc without an agreement.
Mr Grayling said: “I would hope and expect the EU do the same.”
Mrs May will try to sell her Brexit vision to the party’s grassroots today.
She will speak to the Conservative national convention, where she is expected to face tough questions over her Brexit plans.
Sources denied Mrs May had decided to attend the event following calls to defend her policy to her party.