Sir Malcolm Rifkind has said it is "looking inevitable" that Priti Patel will be sacked for her conduct.
The International Development Secretary has been summoned from a three-day tour of Africa by the Prime Minister to account for holding unauthorised meetings with Israeli officials.
While it was known by Number 10 she had met several politicians while on holiday in Israel, it emerged she had held further gatherings with key figures in Israel's government on Tuesday.
With speculation mounting she will be sacked over this, Sir Malcolm Rifkind - the Foreign Secretary under Margaret Thatcher - said the failure to mention the latest meetings to Theresa May would be "unforgivable."
He told Julia Hartley-Brewer: "It’s looking inevitable.
“The issue wasn’t just about the meetings, it was on business not just to do with her own department.
"Most important of all, she didn’t inform the British government or the ambassador in the country in which they were taking part.
"The decisive consideration - if it’s true - is being reported today she didn’t think it appropriate to mention that, after having returned from Israel, she had two further meetings with Israeli officials in London.
"That simply would be unforgivable."