Cabinet Office minister David Lidington has told MPs that "legally-binding" changes that "strengthen and improve" the Withdrawal Agreement and Political Declaration have been secured.
Mr Lidington said the Attorney General would be publishing advice ahead of Tuesday's debate, but needed time to consider the new changes.
"The Attorney General will publish his legal opinion," he said. "That will be available in good time before the debate.
"I think the house would expect the Attorney General to consider very carefully rather than rush an opinion out to meet the deadline for this statement this evening."
Theresa May had been meeting in Strasbourg with Jean-Claude Juncker and Michel Barnier.
In Dublin, cabinet ministers are due to meet tonight to discuss this development.
Taoiseach Leo Varadkar delayed a visit to the United States for St Patrick's Day as the emergency meeting was called for.