A lawyer-led inquiry into bullying allegations against Mr Raab was widely expected to be published today.
But it is now understood that will not happen, prompting opposition parties to accuse the PM of "dithering".
Mr Sunak needs to decide whether Mr Raab, one of his closest allies, has broken ministerial rules and must be sacked or resign.
The PM received the inquiry's findings at around 11:30 BST, with No 10 saying earlier it would be published "as swiftly as possible".
Senior figures had also been briefed to expect a decision on the same day the report was received.
The BBC has been told Mr Raab has seen the full report, but has not spoken to the prime minister.
Mr Raab, who is also justice secretary, denies bullying staff and says he always "behaved professionally". He is facing eight formal complaints about his behaviour as a minister.
Senior lawyer Adam Tolley KC was appointed by the prime minister to investigate the allegations in November. But it will be for Mr Sunak to decide whether Mr Raab has broken ministerial rules and what action to take.
Somebody who advised Mr Raab in a senior role in one department told the BBC: "This waiting only extends the anxiety for those who were brave enough to step forward and speak out, particularly those who have had to continue working with Raab at the Ministry of Justice.
"The PM's prevarication makes it feel more likely that the whole thing, the last five months of agony for Raab's subordinates, will end in a whitewash."
Shadow attorney general Emily Thornberry called on Mr Sunak to "stop dithering and delaying" over Mr Raab's fate.
"If he's a bully, he should go - and the prime minister really should be able to read the report, make up his mind, and get on with it," she added.
The Liberal Democrats also accused Mr Sunak of "dither and delay".
The party's chief whip Wendy Chamberlain said: "It feels like almost every week there is an issue with sleaze and scandal where Rishi Sunak is either implicated in himself or too weak to get to grips with it."
And Dave Penman, the boss of the FDA union representing senior civil servants, said making those who raised complaints wait another day showed the system was a "complete farce".
He said: "No-one knows what is going to happen now, there are no rules associated with any investigation, there are no rights for anyone who raises a complaint.
"People have just got to sit and watch telly to try and find out what has happened about very serious bullying allegations they might have made."
Speaking to the BBC's Newscast, Sir Alex Allan - who conducted an inquiry into bullying allegations against former Home Secretary Priti Patel - said the delay suggested the findings of the report could not be "completely clear cut".
"Otherwise he [the prime minister] would have come out with a decision one way or the other," he added.
The former ethics adviser also said having the prime minister decide if Mr Raab had breached rules, rather than the author of the report or the No 10 ethics adviser, was not "very satisfactory".
There are conversations taking place in government circles about what will happen next if the justice secretary leaves his position.
If he resigns, or is sacked, that would trigger a reshuffle of Mr Sunak's cabinet.
Some senior figures in government are bracing for the report to be damning, and feel he might have no choice but to go.
However, the ultimate decision lies with the prime minister and a final judgement has not been made yet.
The complaints against Mr Raab, involving at least 24 people, related to his previous periods as justice secretary and foreign secretary under Boris Johnson and his time as Brexit secretary under Theresa May.
The MP for Esher and Walton was sacked as justice secretary and deputy prime minister when Mr Johnson was succeeded by Liz Truss.
However, he was reappointed to the two roles when Mr Sunak entered Downing Street in October.
The prime minister has been under pressure to explain what he knew about the allegations before reappointing Mr Raab to the cabinet.
He has repeatedly declined to say whether he had informal warnings about Mr Raab's behaviour before bringing him back into government.