Boris Johnson's senior aide Dominic Cummings said he has "no regrets" about making a 260-mile trip to County Durham during lockdown.
He said his decision was based not only on fears over a lack of childcare if he and his wife were both taken ill with Covid-19 but also concerns about his family's safety.
At a press conference in Downing Street's rose garden, Mr Cummings insisted: ''I believe in all the circumstances I behaved reasonably and legally.''
He told reporters that he had received threats of violence, with many people on social media encouraging attacks directed towards him, so he felt unsafe in his London family home.
So when his wife fell ill and he was concerned there could be a lack of childcare for their four-year-old son if he was also sick, he decided to drive to Durham where his parents own an empty cottage in the grounds of their farmhouse and his sister and nieces live nearby.
''My tentative conclusion on the Friday evening was this: if we were both unable to look after our child then my sister or nieces can look after him.''
He added: ''But, I thought, if I do not develop symptoms and there's a testing regime in place at work I could return to work if I tested negative. In that situation I could leave my wife and child behind in a safe place."
Mr Cummings said he became bedbound himself while in Durham with Covid-19 symptoms and he later took a second shorter trip to Barnard Castle to see if he was up to making the five-hour return journey home. He denied visiting the castle and said they didn't come into contact with anyone.
He said he didn't tell Mr Johnson about his plans at the time but admits that, with hindsight, he should have done.
Setting out the details of his actions, Mr Cummings said the Prime Minister had asked him to publicly give his account and he acknowledged he should have spoken earlier.
''I know that millions of people in this country have been suffering, thousands have died, many are angry about what they have seen in the media about my actions,'' he said. ''I want to clear up the confusions and misunderstanding."
On Sunday, Mr Johnson said he believed Mr Cummings had "no alternative" but to travel from London to the North East for childcare "when both he and his wife were about to be incapacitated by coronavirus".
"In every respect, he has acted responsibly, legally and with integrity," Mr Johnson said.
Before today's statement from Mr Cummings, Alastair Campbell, former spin doctor to former Labour Prime Minister Tony Blair, tweeted: ''Cummings speaking in the Rose Garden is like Monica Lewinsky giving her side of the story from the Oval Office. ''Heaven knows how the cabinet secretary has allowed this. But hey - in Trump Britain all norms rules and standards are collapsing.''
The statement came after it was revealed that Durham Police have formally been asked to "establish the facts" around Mr Cummings' visit and to establish whether he broke the law, The Mirror has confirmed.
On Monday, May 25 Police Commissioner Steve White wrote to the force's Chief Constable, Jo Farrell, questioning if any rules were broken when Mr Cummings visited County Durham at the end of March.
Retired teacher Robin Lees, 71, reported Mr Cummings to the police over the weekend following claims that he he saw the top aide out in Barnard Castle - 30 miles from where he had stayed for two weeks - on April 12.
A witness also told the Sunday Mirror they saw Mr Cummings walking in County Durham on April 19, days after he returned to work in London, but ministers have denied he visited a second time.
It's now in the hands of the police as to whether an investigation will be launched.