The prime minister says his predecessor asked him to nominate people despite a panel rejecting them.
Rishi Sunak has accused Boris Johnson of asking him to “overrule” a panel tasked with vetting his appointments to the House of Lords.
The House of Lords Appointments Commission (HOLAC) has confirmed it rejected eight of the former prime minister’s nominations.
Mr Sunak said Mr Johnson asked him to overrule them, or “make promises to people”.
But he said he refused, adding it was “something I wasn’t prepared to do”.
“I wasn’t prepared to do that, I didn’t think that was right. And if people don’t like that, then tough,” he told a tech conference in London.
The intervention marks a new point in an escalating war of words over Mr Johnson’s controversial resignation honours list.
Mr Johnson announced he was quitting Parliament on Friday over a report from the Privileges Committee, who were investigating whether he had misled MPs about lockdown parties in Downing Street.
It came hours after Downing Street published his resignation honours list without the names of key supporters, including Nadine Dorries, Sir Alok Sharma and Nigel Adams.
All three had been expecting to be appointed to the House of Lords. Competing claims have now surfaced about how and why the names were removed.
As a departing prime minister, Mr Johnson has the right to nominate people for seats in the House of Lords, and for others honours such as knighthoods.
By convention, current prime ministers pass on the list of nominees to HOLAC, which can recommend their names do not go forward after a vetting process.
A source familiar with the process has told the BBC that Mr Sunak’s political team removed some of Mr Johnson’s suggestions months ago.
However, Downing Street has insisted Mr Sunak passed on Mr Johnson’s list of nominations unaltered.
It says it also accepted HOLAC’s full approved list and passed it to the King.
The BBC has has asked Mr Johnson for a comment.