Several senior Labour figures claimed that McSweeney had been resisting the inevitability of Mandelson’s departure on Wednesday, with one insider describing “cold, hard fury” amongst those in Downing Street about the episode.
However, another senior Downing Street source claimed this was nonsense, saying that by Wednesday afternoon McSweeney was adamant that Mandelson’s position was untenable.
A government minister said they were “starting to wonder how sustainable it is” for McSweeney to stay in post.
One Labour MP said: “Panic has started to set in”, urging the prime minister to “get a grip” and warning that only publishing correspondence between No 10, McSweeney and Lord Mandelson before his appointment as ambassador would “put this to bed”.
Another said the handling of the situation had been “a shambles”.
Speaking to BBC Breakfast, Scottish Secretary Douglas Alexander said: “In retrospect, of course, if (it) had been known at the time what is known now, the appointment wouldn’t have been made.”
Acknowledging it had been a difficult week for Labour he said: “Many of us were devastated by [deputy PM] Angela Rayner’s departure from the government last week.
“She’s an extraordinary woman who’s overcome the most extraordinary challenges and we are grieving and feel quite acutely that sense of loss.
“Now to have the dismissal of Peter Mandelson just the next week, I totally get it, of course Labour MPs will be despondent that in two weeks in a row we have seen significant resignations from public service.
“These are not the headlines any of us in government or in Parliament would have chosen or wanted.
“But the fact is when the evidence emerged, action had to be taken and we are looking forward, therefore, to moving on.”
Conservative frontbencher Alex Burghart said his party would force a vote in Parliament to release the documents that the prime minister and the foreign secretary were shown before appointing Lord Mandelson.
“Those documents exist, they will be on file… it’s inconceivable they would not have been shown concerns raised by the security services through the vetting process,” he told BBC Breakfast.
The Liberal Democrats have said there should be a review of vetting procedures.
Some Labour MPs have publicly expressed anger at how the situation with Mandelson has been handled.
Paula Barker – who dropped out of the deputy Labour leader race on Thursday – said: “The delay in sacking him has only served to further erode the trust and confidence in our government and politics in the round.”
Charlotte Nichols said Mandelson’s sacking was “not immediate enough unfortunately, as he should never have been appointed in the first place”.
Sadik Al-Hassan said there were “serious questions about the vetting process of the ambassador”.