Boris Johnson has said he did not discuss government business at a meeting with ex-KGB officer Alexander Lebedev "as far as I am aware".
The prime minister is facing questions about why no officials were present to take notes at the 2018 meeting with the Russian oligarch.
The PM – who was foreign secretary at the time – says it was a private social event that had not been pre-arranged.
Labour said his "mealy-mouthed statement" raised more questions.
The Soviet KGB was the main Cold War rival of Western security services and President Vladimir Putin – along with some of his closest comrades – worked for the agency.
Mr Lebedev's son Evgeny Lebedev, the owner of the London Evening Standard and a shareholder in The Independent, was given a seat in the House of Lords by Mr Johnson in 2020.
The appointment has been shrouded in controversy after The Sunday Times newspaper alleged security services withdrew an assessment that the peerage posed a national security risk after the prime minister intervened.
Mr Johnson has previously denied this and Lord Lebedev has rejected the suggestion he posed a "security risk" to the UK.
In an article, he acknowledged his father "was a foreign intelligence agent of the KGB", but insisted he was not "some agent of Russia" and condemned its invasion of Ukraine.
Labour raised concerns about Lord Lebedev's appointment.
Parliament's Intelligence and Security Committee demanded documents about the process of giving him a seat in the Lords.
But ministers withheld information from Parliament citing national security – despite MPs voting for it to be released.
Meanwhile, Mr Johnson has been under mounting pressure to explain the nature of his meeting with Mr Lebedev's father Alexander in April 2018, at the oligarch's Italian home.
Mr Johnson has been criticised for not having security officers with him on the trip, which took place after a Nato summit at which Russia was on the agenda.
Weeks earlier a former Russian military officer and his daughter were poisoned in the English city of Salisbury, in an attack the UK blamed on the Kremlin.
The prime minister confirmed the meeting with Mr Lebedev, who bought the Evening Standard with his son in 2009 but later stepped down from the company, at the Commons Liaison Committee earlier this month.
Mr Johnson was asked to provide further written answers on a number of questions after the appearance in front of senior MPs.
In a letter, Mr Johnson said he declared the overnight stay at Mr Lebedev's house "in the interests of transparency" and insisted the trip was "in line with established security protocols".
"It would not have been normal practice for civil servants or security staff to have accompanied me to such a private, social occasion," he wrote.
He also noted he took no ministerial papers with him, adding: "As far as I am aware, no government business was discussed."
Labour said the prime minister's letter raised "more questions than it answers" because "he apparently still can't recall whether he discussed government business or not" and it suggested the PM "has something to hide".
The party's deputy leader Angela Rayner said: "Keeping the British people safe should be a priority of government, but this web of murky relationships shows the Conservatives cannot be trusted with our national security."
Boris Johnson met Russian oligarch without aides
PM was warned of Lebedev concerns, says Cummings
I’m not a UK security risk, Russian-born peer says
Ukraine steps up bid to retake Kherson – sources
Kim says N Korea ready to mobilise nuclear forces
Million in Wuhan lockdown after four Covid cases
Thai cave rescue: 'At first it was difficult' Video
The secret mission to save LGBT Afghans
West's modern weapons stifle Russia's firepower
'My most prized possession is gone' Video
Climate change killing elephants, says Kenya. Video
It's Birmingham's turn to shine – Peaky Blinder creator
Seven things you might not know about the Commonwealth
Why England is not the Premier League's top market
The habits that help prevent allergies
Why we brush our teeth wrong
Most of us don't clean our teeth in the right way
The animation too dark for Hollywood
Why dark Japanese fairy tale Princess Mononoke was too much for Hollywood
Delhi's opulent 'snack of wealth'
Some street vendors say moonlight and dew are the magic ingredients
© 2022 BBC. The BBC is not responsible for the content of external sites. Read about our approach to external linking.
