PM’s main assistant, Dominic Cummings, may have broken the lockout rules, but it would have been a “minor offense,” said Durham police.
Boris Johnson said he intended to “put an end to the case” following the police statement.
And he told reporters at the daily Downing Street briefing that he did not want his scientific advisers to be drawn into a political line.
Opposition parties continue to demand the removal of Mr. Cummings.
Johnson said at the Downing Street press conference, “I have said a lot about this before and what I also note is that the Durham police said they were not going do nothing and the case was closed.
“And I intend to draw a line on this. “
The Prime Minister intervened to prevent his chief medical adviser, Chris Whitty, and his chief scientific adviser, Sir Patrick Vallance, from answering questions from reporters on the actions of Mr. Cummings.
He told BBC political editor Laura Kuenssberg, who asked councilors if Mr. Cummings set a good example, that he would intervene.
He said he wanted to “protect them from what I think is an unjust and unnecessary attempt to ask political questions.”
“It is very, very important that our doctors and scientific advisers are not trained in what I think most people recognize as fundamentally a political argument,” he added.
Later in the briefing, Professor Whitty told another reporter that he had no desire to “get drawn into politics”, a sentiment immediately echoed by Sir Patrick.
It was the first appearance of the two senior advisers at a briefing in Downing Street since the story of Dominic Cummings broke out last Friday night.
Labor leader Sir Keir Starmer said Johnson should have allowed his advisers to answer questions from reporters, adding, “We want transparency.”
“No one should be prevented from answering questions from journalists … but it is the Prime Minister here who is at fault, he has been too weak everywhere,” said the Labor leader.
He said he would have sacked Mr. Cummings if he had been prime minister, and that there had been a week of unnecessary “distraction” from the safe relaxation of the lockdowns because Mr. Johnson “was frankly too weak to draw a line below and take the necessary action. ”
By Helen Catt, BBC News political correspondent
This is a time of maximum risk for the government and it is coming to the end of a difficult week.
Boris Johnson clearly wanted to allay concerns about when to announce his easing. So we had a methodical briefing with graphs and statistics to show how these “five tests” are met.
But he also wanted to reinforce the message that the virus is still a threat. The government will have to hope that it has balanced this balance correctly.
The backdrop to all of this was, of course, the argument over the Prime Minister’s adviser, Dominic Cummings, and whether his trip to Barnard Castle could make this more difficult.
And this is where the briefing took a sharper turn.
For the first time at these press conferences, the Prime Minister took the extraordinary step of preventing the advisers accompanying him from answering questions from journalists.
He said it was to protect them (and they said they didn’t want to be trained). Critics are likely to have a different opinion.
Mr. Johnson clearly wants to draw a line on the events of the past week; the next few days will decide if this happens.
Cummings ‘March decision to drive from his home in London to his parents’ farm in County Durham with his wife – who had symptoms of coronavirus – and his son has hit the headlines since the story broke out on Friday evening.
PM’s chief counselor gave a press conference on Monday, explaining that he decided to travel because he thought it would be best to isolate himself in a place where he had childcare options. children if necessary, and insisted that he had acted “reasonably”. and in the law.
He said he made the 50-mile round trip to Barnard Castle with his wife and child 15 days later to test his eyesight before embarking on the longest trip back to London.
In a statement released earlier on Thursday, Durham police said they “did not consider an offense to have been committed” when Mr. Cummings drove his family from London to Durham to isolate themselves in the family farm.
His subsequent trip to Barnard Castle “could have warranted police intervention,” added the force, but she would take no action against him now.
Durham police said they considered the likely breach of the lockdown rules at Barnard Castle to be minor as there were “no apparent violations of the social distancing.”
“If a Durham Constabulary police officer had prevented Mr. Cummings from driving to or from Barnard Castle, the officer would have spoken to him and, having established the facts, would likely have advised Mr. Cummings to return to the address of Durham, providing advice on the dangers of travel during the pandemic crisis.
“If this advice had been accepted by Mr. Cummings, no enforcement action would have been taken. “
The force said it would not take retrospective action against Mr. Cummings because it would amount to “treating Mr. Cummings differently from other members of the public.”
Reports that Mr. Cummings was seen in Durham on April 19, suggesting a second trip from London, could not be supported by evidence, the police statement added.
Mr. Cummings denied being in Durham on that date.
BBC correspondent Danny Shaw said it was understood that Durham police had investigated other aspects of Mr. Cummings’ stay in the area, including a possible violation of traffic safety laws and his travels to and from a hospital, where his son was treated.
“His findings are not included in the press release – suggesting that the force may not have been able to draw a line in the case,” added our correspondent.
Forty Conservative MPs called on Mr. Cummings to resign or be fired for undermining the government’s lockdown message and the Prime Minister’s authority.
SNP Westminster chief Ian Blackford said that Johnson “had no choice but to remove him from office.”
And the Liberal Democrats’ acting leader, Sir Ed Davey, said that “the only way” for the Prime Minister to get out of the controversy “would be to demand the resignation of Dominic Cumming.”