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.
Ms Patel's replacement is expected to be announced later in the day.
The row began last week, when the BBC revealed Ms Patel held a number of meetings with business and political figures during a family holiday to Israel in August.
After the visit, she suggested some of Britain's aid budget go to the Israeli army and asked her officials to see if Britain could support humanitarian operations conducted by the Israeli army in the occupied Golan Heights area.
She was formally reprimanded in Downing Street on Monday, where she was asked to give details about the meetings - which were not sanctioned by the Foreign Office.
Ms Patel, who has served as the Tory MP for Witham in Essex since 2010, was then forced to correct the record about the number of meetings she attended and when the Foreign Office had been notified.
She admitted she had been wrong to suggest Foreign Secretary Boris Johnson knew of the trip in advance.
Later, it emerged she held two further meetings in September without government officials present.
She was accused of breaching the ministerial code - which sets out the standards of conduct expected of government ministers.
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 he understood Ms Patel had met UK officials during the holiday.
"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.
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