The BBC has said an introduction about Dominic Cummings on Tuesday's Newsnight did not meet the required standards of due impartiality.
The programme began with presenter Emily Maitlis saying "the country can see" he had "broken the rules".
It should have made clear the remarks were "a summary of the questions we would examine" about the prime minister's aide, the corporation said.
The BBC said the news programme's staff had been reminded about its guidelines.
At the beginning of the BBC Two programme, Maitlis said the country was "shocked" that the government could not see that Boris Johnson's aide had broken the rules by travelling from London to County Durham during the coronavirus lockdown.
She said the "public mood" was "one of fury, contempt and anguish", and that Mr Cummings had made people who struggled to keep to the government's rules "feel like fools".
She continued: "The prime minister knows all this. But despite the resignation of one minister, growing unease from his backbenchers, a dramatic early warning from the polls and a deep national disquiet, Boris Johnson has chosen to ignore it.
"Tonight we consider what this blind loyalty tells us about the workings of Number 10."
In a statement on Wednesday, the BBC said it had "reviewed the entirety of last night's Newsnight, including the opening section".
"While we believe the programme contained fair, reasonable and rigorous journalism, we feel that we should have done more to make clear the introduction was a summary of the questions we would examine, with all the accompanying evidence, in the rest of the programme," it continued.
"As it was, we believe the introduction we broadcast did not meet our standards of due impartiality."
Mr Cummings' 260-mile journey has been the focus of intense media scrutiny since coming to light last week.
On Monday, the prime minister's most senior adviser explained that he decided to make the trip because he felt it would be better to self-isolate in a place where he had options for childcare if required.
On Wednesday, Boris Johnson ruled out an inquiry into his adviser's conduct, insisting it was time to "move on" from the row.