Keir Starmer is expected to admit he inadvertently gave MPs misleading information about Peter Mandelson’s vetting when he addresses the Commons.

But his spokesperson said the prime minister would “never knowingly mislead parliament or the public” and that he was himself misled.

Starmer is to address parliament on Monday afternoon after days of pressure over the Guardian’s revelation that Mandelson failed secured vetting before he took up his post as US ambassador.

Starmer has said three separate times in the Commons that “full due process” was followed, prompting opposition leaders to say he misled parliament and should correct the record.

His official spokesperson said: “This information should have been provided to parliament. It should have been provided to him [Starmer], it should have been provided to other government ministers …

“But he clearly did not have this information – that is the crucial fact – he clearly did not have this information when he previously spoke to parliament.”

Downing Street said Starmer would not have appointed Mandelson had he been aware that United Kingdom Security Vetting (UKSV) had recommended against his security clearance, but it did not say whether this position had been conveyed to the top civil servant at the Foreign Office, Olly Robbins.

Starmer’s admission that he had not been aware of Mandelson’s failed security vetting has been greeted with shock and incredulity across Westminster, given the sensitivity of the Washington ambassador posting.

It has already led to the sacking of Robbins, who is expected to appear before a select committee of MPs on Tuesday in what could be a moment of grave peril for Starmer.

The prime minister has said he will make it “crystal clear” to MPs he had been left in the dark over Mandelson’s vetting, and that it was “unforgivable” the Foreign Office failed to tell him of the situation after he had told MPs due process had been followed.

Starmer will set out the facts on Mandelson’s security vetting after he instructed officials to urgently establish them when he was finally informed about the decision last Tuesday.

Downing Street has sought to demolish the argument from Robbins’ allies that he was prevented by law from telling ministers that Mandelson had failed vetting, with No 10 arguing there was a difference between being involved in the decision and being informed of it. It suggested Robbins should have been aware of the expectations on sharing information that are set out in the civil service code.

However, the then cabinet secretary, Simon Case, appeared to advise Starmer to complete security vetting for Mandelson before announcing any appointment, documents have revealed.

The documents released last month by the Cabinet Office as part of the disclosures over the appointment also show Mandelson was offered a “higher tiers” briefing before his vetting was finalised.

Starmer’s spokesperson said: “As is normally the case with external appointments, both to the Foreign Office and to the wider civil service, appointments are made subject to obtaining security clearance.” Downing Street has now stopped that process.

Downing Street said it and the Cabinet Office had asked “repeated questions” of the Foreign Office “in order to be assured that the proper process was followed” during Mandelson’s appointment, although did not say why it had done so, or whether this was a result of concerns.