A phone call that Foreign Secretary Dominic Raab was advised to make but which was given to a junior minister to handle did not happen, it has emerged.
The government had said the call to get help evacuating interpreters from Afghanistan had been delegated as Mr Raab was busy on other calls.
However, a spokesman now says it "was not possible to arrange a call" before the Afghan government collapsed.
Mr Raab is facing pressure over his handling of the Afghanistan crisis.
Opposition parties called for him to resign after it was revealed he was unavailable to make the phone call last Friday while he was on holiday, as the Taliban advanced towards Kabul, Afghanistan's capital.
It followed newspaper reports suggesting the foreign secretary was seen relaxing on a beach on the Greek island of Crete on Sunday – the day Kabul was captured.
Cabinet colleagues have defended Mr Raab, who has rejected calls to quit, but some Conservative MPs remain unhappy with how he has dealt with the UK response to the escalating crisis.
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On Thursday, it was reported he was advised by senior Foreign Office officials last week that he should make contact with Afghan Foreign Minister Hanif Atmar to get urgent assistance in rescuing Afghan interpreters who had worked for the British military.
Officials said it was important the call was made by Mr Raab rather than a junior minister – but they were told he was unavailable. The Afghan foreign ministry reportedly refused to arrange a call with a junior minister, pushing it back to the next day.
And, as reported in the Daily Mail, the Foreign Office has now confirmed the call did not take place.
A spokesman told the BBC: "Given the rapidly changing situation it was not possible to arrange a call before the Afghan government collapsed."
Labour has set out a list of 18 urgent questions for the foreign secretary to answer about his trip and his department's handling of the crisis.
The party said it was requesting specific answers on when Mr Raab was out of the country and on leave from official duties and if he received advice from officials on the advisability of leaving as the situation in Afghanistan deteriorated.
The party also questioned the prime minister's involvement, asking Mr Raab if he spoke with Boris Johnson while he was away, and whether the PM had given permission for him to leave the country.
Labour's shadow foreign secretary Lisa Nandy said: "For the prime minister and foreign secretary to be on holiday during the biggest foreign policy crisis in a generation is an unforgivable failure of leadership."
Mr Raab earlier this week insisted he was "engaged in Cobra, talking to foreign counterparts" and speaking to his team in London "on an hour-by-hour basis" while he was away and he left "as soon as the situation deteriorated and demanded it".
Among those defending Mr Raab was Health Secretary Sajid Javid, who said his cabinet colleague was "one of the most professional and most effective ministers I've ever worked with".
media captionThe foreign secretary gives a one word answer when asked about his future.
On Thursday, Mr Raab chaired a call of G7 foreign ministers to discuss Afghanistan, saying afterwards that efforts "to do everything possible to evacuate vulnerable persons" from Kabul airport were ongoing.
Meanwhile, the Times has reported that the permanent secretaries – the top civil servants – at the Home Office, Foreign Office and Ministry of Defence are currently on holiday.
In response, a government spokesman said that "departments across Whitehall have been working intensively" on the situation in Afghanistan.
The BBC has been told that the civil servants in question are continuing to work while on holiday, and that the Ministry of Defence and Home Office have two permanent secretaries each, which ensures one is at work when the other is on leave.
The Foreign Office has a designated acting permanent secretary to cover periods of leave.