Mandelson, 72, was sacked as US ambassador in 2025, just nine months into the Washington DC posting, after further details of his association with late sex offender Jeffrey Epstein emerged.
In a House of Commons statement, Starmer will be faced with allegations he misled parliament after telling MPs the proper process had been followed in appointing Mandelson.
Starmer has insisted he was kept in the dark about the peer being red-flagged by security experts.
The prime minister effectively fired the Foreign Office's top official Olly Robbins last week after it emerged Mandelson had been given developed vetting status despite failing checks carried out by the agency responsible for assessing security clearances.
The scandal has fuelled calls for Starmer to resign, both from opposition parties but also from his critics within the Labour movement.
The prime minister's defence will be to blame officials for not telling him or the then foreign secretary David Lammy that UK Security Vetting had not cleared Mandelson.
A statement issued by No.10 Downing St on Sunday night, local time, said that although civil servants rather than ministers make decisions on vetting and clearance, there was nothing in the law to prevent ministers being told.
"There is nothing in the guidance which prevented information being shared in this scenario, in a proportionate and necessary way and subject to the appropriate procedural steps," the statement said.
Starmer told the Mirror he would make it clear to MPs he had been kept in the dark and it was "unforgivable" that the Foreign Office failed to tell him after he had offered public assurances that proper process had been followed.
"The fact that I wasn't told that Peter Mandelson had failed his security vetting when he was appointed is astonishing," Starmer said.
"The fact that I wasn't told when I said to Parliament that due process had been followed is unforgivable, and that's why I intend to set out in Parliament on Monday the facts behind that, so there's full transparency in relation to it.
"But am I furious that I wasn't told? Yes, I am. Am I furious that other ministers weren't told? Yes, I am. I should have been told, and I wasn't told."
Ed Davey, the Liberal Democrats leader, said Starmer had shown "catastrophic misjudgement" while Conservative leader Kemi Badenoch said her opponent is "either lying or he's incompetent".
"This has been a tawdry and shaming affair for you and your party, and for this country," Badenoch said in a letter to the prime minister.
"Not only have you damaged our relationship with the United States and insulted the victims of the pedophile Jeffrey Epstein, but you have also undermined our national security by giving the highest diplomatic post to an individual that the security services found to be of 'high concern'."
With Reuters
