Theresa May is facing a second cabinet reshuffle in a week, as she comes under pressure to say what the Foreign Office knew of Priti Patel’s visit to Israel.
Ms Patel resigned as international development secretary on Wednesday after holding a series of unauthorised meetings with Israeli officials.
In a letter to the PM, she said she had lacked “transparency and openness”.
However, Labour has now called on Mrs May to say when government officials knew about the undisclosed meetings.
Ms Patel resigned having been told by the prime minister to return from an official trip in Africa and report to Downing Street.
It is the second cabinet resignation in the space of seven days, after Sir Michael Fallon quit as defence secretary last week. He was replaced by Gavin Williamson, as Mrs May adjusted her government team.
A replacement for Ms Patel, who was a key figure in the Vote Leave campaign, is expected to be announced on Thursday, with a lot of attention on whether or not it goes to a Brexiteer.
The row began last week, when the BBC revealed Ms Patel arranged a number of private meetings with business and political figures during a family holiday to Israel in August.
After the visit, she asked her officials to look into whether Britain could support humanitarian operations conducted by the Israeli army in the occupied Golan Heights area.
But Foreign Office officials strongly advised against this as the need for humanitarian aid was greater elsewhere and giving aid to the military broke aid rules, BBC diplomatic correspondent James Landale said.
Ms Patel, who has served as the Tory MP for Witham in Essex since 2010, was formally reprimanded in Downing Street on Monday.
She was asked to give details about the meetings, which were not sanctioned by the Foreign Office, and to correct information she initially gave when details of the meetings were published.
It later emerged that she had held two further meetings in September with Israeli public security minister Gilad Erdan in Westminster and Israeli foreign ministry official Yuval Rotem in New York – again without government officials present.
Asked if Ms Patel had been foolish or had made a concerted attempt at freelance foreign policy, the BBC’s James Landale told BBC Radio 4’s Today: “I think it’s pretty clear that the view within the government is there was an attempt to try to shape British policy within the Middle East.”
Labour has now called on the government to set out what Foreign Office officials knew of the meetings.
Writing to Mrs May, Labour’s deputy leader Tom Watson said: “I have been informed that while she was in Israel, Ms Patel met officials from the British consulate general Jerusalem, but that the fact of this meeting has not been made public,” he wrote.
“If this were the case, then it would surely be impossible to sustain the claim that the Foreign and Commonwealth Office was not aware of Ms Patel’s presence in Israel.”
He added: “The existence of such a meeting or meetings would call into question the official account of Ms Patel’s behaviour, and the purpose of her visit.”
Middle East minister Alistair Burt told MPs on Tuesday that Foreign Office officials in Israel were made aware of Ms Patel’s visit on 24 August and it was likely that her meetings had taken place beforehand.
Ms Patel was accused of breaching the ministerial code – which sets out the standards of conduct expected of government ministers.
In her resignation letter, Ms Patel said: “While my actions were meant with the best of intentions, my actions also fell below the standards of transparency and openness that I have promoted and advocated.
“I offer a fulsome apology to you and to the government for what has happened and offer my resignation.”
In her reply, Mrs May said: ”As you know the UK and Israel are close allies, and it is right that we should work closely together. But that must be done formally, and through official channels.
”That is why, when we met on Monday I was glad to accept your apology and welcomed your clarification about your trip to Israel over the summer.
“Now that further details have come to light it is right you have decided to resign.”
Conservative MP Jacob Rees-Mogg suggested disgruntled Remainers could be behind the leak that led to the downfall of Ms Patel, who is a prominent Brexiteer.
He told BBC’s Newsnight that some people were “still very bitter” about the referendum result and “inevitably that colours their behaviour”.