The letter, seen by ITV News, says she is allowing the access “as an exceptional measure”, pointing to the “particular circumstances surrounding this set of events”.
Dated 17 February, it reads: “I appreciate that this is a worrying time for those affected by this process, which I do not wish to compound,” ITV said.
The move – revealed ahead of Friday night’s deadline for Mr Johnson to submit his legal questionnaire – means he and all those under investigation will know what information the police hold on them, before responding.
It raises questions about whether, if an individual learns there is nothing incriminating in the notes on them, he or she will volunteer any additional information to the Met.
Mr Johnson is believed to be working in Downing Street today, ahead of flying to the Munich security conference on Saturday to deliver a speech on Russia’s threat to Ukraine.
Ms Gray has set strict conditions on the access allowed, including that it must be “with a member of the investigation team present” and be “time limited”.
“You will, in line with the process for investigations of this kind and in keeping with the interview process, not be allowed to bring any legal representative with you.” ITV said she has written.
“You will not be permitted to bring phones, tablets, computers or any other recording equipment into the room with you.
“You will not be permitted to challenge, suggest changes or amendments to the notes or otherwise challenge their contents.”
Mr Johnson is still fighting for his political life and will come under huge pressure to quit if it is confirmed he attended, or knew about, any parties that broke the law.
He has hired a personal lawyer to help him draft his response to questionnaire, in which he will argue it was part of his working day when he attended as many as six different gatherings.
The prime minister is in greatest danger over the “bring your own bottle” party in the No 10 garden, in May 2020, which he has admitted attending – while claiming he did not realise it was a party.
He has also not denied attending the “Abba party” in his flat in November 2020 – to celebrate the departure of Dominic Cummings – and appears to be preparing to argue he was working while it went on.
Last month, Ms Gray passed her evidence – including around 300 photographs – to the Met, which launched a separate investigation that has delayed the publication of her report.
Questionnaires have been sent to around 80 people under investigation, who may be issued with fixed penalty notices if they have broken Covid rules.
Staff will not be allowed to view any information Ms Gray gathered on anyone except themselves.