The PM’s chief adviser Dominic Cummings is to make a public statement at 16:00 BST over allegations he broke coronavirus lockdown rules.
Mr Cummings is facing calls from Labour and some Tory MPs to quit or be fired.
He travelled 260 miles with his family to be near relatives when his wife developed Covid-19 symptoms.
Boris Johnson insists his aide acted legally and within guidelines – but critics say the government’s lockdown message has been undermined.
The prime minister made a statement on Sunday in an attempt to draw a line under the row – but Conservative MPs have continued to call for Mr Cummings’ dismissal.
Mr Cummings has been under fire since the Guardian and Daily Mirror reported that he had been seen in County Durham, at his family’s farm during lockdown.
It later emerged that he had travelled there from London with his four-year-old son and wife, who had developed Covid-19 symptoms, so that he could self-isolate near relatives who could take care of the child if necessary.
Under lockdown rules, which are still in force, anyone developing symptoms has been instructed to stay in their home.
The Observer and Sunday Mirror reported that Mr Cummings had also been seen in Barnard Castle, 30 miles from his family’s Durham home, on Easter Sunday, at a time when the government was warning people not to travel to tourist spots.
Mr Johnson did not deny that Mr Cummings had gone to Barnard Castle, at his Sunday press conference, but insisted that some press reports of his movements during lockdown were “palpably false”.
Mr Cummings has been quizzed by reporters outside his London home since the row erupted, telling them he had done the “right thing” by making the journey to Durham, and did not care what it looked like.
But he has not yet given a full explanation of his actions – something he will be expected to do when he makes a statement and takes questions from journalists shortly.
It comes as plans to further ease lockdown restrictions are being discussed at a cabinet meeting.
“We’re following the science” has been the phrase often used by ministers. But now some of the very scientists informing government say efforts to tackle the pandemic are being undermined.
Advice on how to build trust and get the public to follow lockdown had been “trashed”, according to one member of the Sage committee on behavioural science.
Maintaining public support is going to be vital in the next stage of dealing with the coronavirus.
The plan will be to replace parts of lockdown with highly targeted testing, contact tracing and persuading people to isolate or quarantine.
Disease modellers at the London School of Hygiene and Tropical Medicine, who are also advising government, say that will require very high levels of public support in order to work.
However, they fear it will now be much harder to achieve.
Twenty Tory MPs are calling for Mr Cummings to resign or be sacked, while others have joined Labour in calling for an inquiry.
Tory MP Peter Aldous said: “My initial view was to be sympathetic to Dominic Cummings due to his expressed desire to protect his young son. I have now revised this opinion.
“I have received many e-mails from constituents highlighting the sacrifices that families have made during the pandemic and expressing upset and anger that there appears to be one rule for those in positions of authority and another for everyone else.
“Moreover, questions remain unanswered as to whether Mr Cummings completely self-isolated whilst he was in County Durham.”
But Tory MP Andrew Bridgen tweeted: “Many have judged Dominic Cummings without hearing his side of the story. That will change this afternoon and will be unprecedented. It’s possible that people will be surprised.”
Scotland’s First Minister Nicola Sturgeon warned the consequences of Mr Johnson’s decision to back Mr Cummings could be “serious”, and acting Lib Dem leader Sir Ed Davey said it undermined the prime minister’s authority on the coronavirus crisis.
Senior Church of England bishops and scientists advising ministers on the pandemic have also strongly criticised the government’s handling of the row.
Mr Johnson has defended Mr Cummings, saying he believed his senior aide had “no alternative” but to make the journey from London at the end of March for childcare “when both he and his wife were about to be incapacitated by coronavirus”.
Durham’s police and crime commissioner, Steve White, has asked the force to “establish the facts concerning any potential breach of the law” surrounding Mr Cummings’ visit to the county.
In a statement, Durham Constabulary said: “We can confirm that, over the last few days, Durham Constabulary has received further information and complaints from members of the public and we are reviewing and examining that information.”