Lord Mandelson was this week stripped of his position as ambassador to the US amid fresh scrutiny over his years-long friendship with the convicted paedophile.
The prime minister initially defended the Labour peer but removed him from his post on Thursday after newly seen emails revealed he sent messages of support to Epstein even as he faced jail for sex offences in 2008.
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The Times has now reported that Downing Street and the Foreign Office were aware of the emails on Tuesday – a day before Sir Keir gave Lord Mandelson his backing at Prime Minister’s Questions (PMQs).
In a post on X, Conservative leader Ms Badenoch wrote: “Looks like the Prime Minister and Labour MPs spent the week lying to the whole country about what they knew regarding Mandelson’s involvement with convicted paedophile Jeffrey Epstein.”
She continued: “If No 10 had those emails for 48 hours before acting, it means he lied at PMQs and ministers lied again about new additional information. These are yet more errors of judgment.
“The Prime Minister has very serious questions to answer. The only way to clear this up is full transparency about who knew what, and when.”
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Sources said Sir Keir was not aware of the contents of the emails when he told MPs he had “confidence” in Lord Mandelson.
A media enquiry outlining details of the messages between Lord Mandelson and Epstein was sent to the Foreign Office on Tuesday, and passed on to Number 10.
Sir Oliver Robbins, the permanent under-secretary at the Foreign Office, asked Lord Mandelson about the emails on Tuesday, but did not receive a response until the following day.
Sir Keir is understood not to have been aware of the contents of the emails until Wednesday evening.
Ms Badenoch’s comments came as Sir Keir enjoyed some time away from Downing Street.
The prime minister was on Saturday pictured watching Arsenal face Nottingham Forest in a Premier League match at the Emirates Stadium.
Lord Mandelson’s exit came after less than a fortnight after another high-profile loss for the Labour government, as Angela Rayner was forced to quit as deputy prime minister and deputy Labour leader over her tax affairs.
As Sir Keir has faced a scandal-hit start to the month, a growing number of Labour MPs have begun calling his leadership into question.
Read more from Sky News:
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Lucy Powell, who is running to replace Ms Rayner as Labour’s deputy leader, has called for a “change of culture” at Downing Street.
“We’ve got a bit of a groupthink happening at the top, that culture of not being receptive to interrogation, not being receptive to differing views,” she told The Guardian newspaper.
Meanwhile, senior Labour MP Emily Thornberry has written to the new foreign secretary, Yvette Cooper, demanding answers about the vetting process for UK diplomats in the wake of Lord Mandelson’s sacking.
