Dominic Cummings might have committed a "minor breach" of lockdown rules when he made a journey to Barnard Castle, Durham Police's investigation has concluded.
Mr Cummings, Boris Johnson's chief aide, made a 260-mile trip to stay close to his parents in Durham when he was ill with coronavirus symptoms during lockdown.
Durham Police "does not consider" an offence was committed by making the journey.
But Mr Cummings made a further trip to check his driving ability following eye sight difficulties which he had suffered while ill.
Durham Police say that in making the 52-mile round journey, there "might have been a minor breach" of coronavirus regulations that "would have warranted police intervention".
The force said the breach is considered minor because there was "no apparent breach of social distancing".
It added: "Had a Durham Constabulary police officer stopped Mr Cummings driving to or from Barnard Castle, the officer would have spoken to him, and, having established the facts, likely advised Mr Cummings to return to the address in Durham, providing advice on the dangers of travelling during the pandemic crisis.
"Had this advice been accepted by Mr Cummings, no enforcement action would have been taken."
The 48-year-old said he had made the trip so he could be closer to childcare should he or his wife become incapacitated by the illness.
Responding to the Durham Police statement, a Number 10 spokesman said: "The police have made clear they are taking no action against Mr Cummings over his self-isolation and that going to Durham did not breach the regulations.
"The Prime Minister has said he believes Mr Cummings behaved reasonably and legally given all the circumstances and he regards this issue as closed."
Durham Police added how there was "insufficient evidence" to support allegations that Mr Cummings made a further trip to Durham on April 19.
The prime minister has resisted sufficient pressure to sack Mr Cummings for making the journey.
During an appearance at the Liaison Committee on Wednesday, Mr Johnson described the story as a "political ding dong" and said we would not be ordering an inquiry.
Labour leader Sir Keir Starmer said Mr Johnson's refusal to sack Mr Cummings showed he was "too weak to act".
Sir Keir, who has said he would have sacked the adviser if he was PM, said: “Boris Johnson should have drawn a line under the Dominic Cummings saga but was too weak to act.
“The public have sacrificed so much for the health of our nation – which he’s now undermined.
“And sent a message that there’s one rule for them and another for the British people.”
Asked by Committee Chair Sir Bernard Jenkin why an inquiry had not been ordered into Mr Cummings' journey, the PM said he is "not certain" looking into the matter would "be a very good use of official time".
After news broke of Mr Cummings' trip, several Cabinet ministers put out statements defending the aide's actions.
The dogged defence of Mr Cummings has resulted in many accusing the government of allowing the lockdown message to be confused.
Labour's Yvette Cooper told Prime Minister Johnson he was "putting political concerns ahead of clear public health messages", an accusation he denied.
"The reason you sent all of your ministers out to say fudgey things and unclear things is because you are trying not to incriminate Dominic Cummings and you don’t want to apologise for him," she added.
He hit back, urging MPs to "lay aside party political points scoring and put national interest first" - he repeatedly said it was time to "move on".
But a poll conducted after the news of Mr Cummings' trip broke, showed some 59% of Britons think he should resign.
And at least 40 Tory MPs have now called for his resignation, including former Scotland minister Douglas Ross, who resigned over the government's response to the news.
Backbench Tories to call for his resignation include Mark Harper, Sir Roger Gale and Steve Baker.