Martin Reynolds is the latest of the prime minister Boris Johnson's closest aides to come under fire for his role in the Partygate scandal.
Since allegations of a number of parties at 10 Downing Street - during a period when the nation was locked down by strict Covid restrictions - were broken by the Mirror last year, some people in power under Johnson's government have stepped down.
After press secretary Allegra Straton resigned over ITV's video leak of her joking about breaking restrictions on a day when 629 people died due to the virus, Martin Reynolds could be the next person to face repercussions.
Police are now investigating reports that No 10 had another lockdown party in May 2020. A leaked email inviting more than 100 Downing Street staff to a bring-your-own-booze party is at the centre of the investigation.
The email was sent by Martin Reynolds - here's what you need to know about the civil servant.
Who is Martin Reynolds?
Martin Reynolds is the Principal Private Secretary to the Prime Minister, which means he acts as the head of the PM's office and in the Civil Service.
Reynolds, who studied law at the University of Cambridge, worked as a lawyer in London before entering politics. Between 2011 and 2014, he served as Deputy High Commissioner at the British High Commission in Pretoria, South Africa.
On leaving that post, Reynolds became Principal Private Secretary to then Foreign Secretary Boris Johnson, until the future PM's resignation in 2018 over a Brexit row with Theresa May.
When Johnson became Prime Minister in 2019, Reynolds left his post as UK ambassador to Libya to take over from Peter Hill, who resigned as Principal Private Secretary.
Just before the January 10 leaked email, Reynolds was expected to secure a senior diplomatic or ambassadorial role, after investigations looking into Downing Street lockdown gatherings intensified.
What was the email sent by Martin Reynolds?
The full email inviting Downing Street staff and the PM himself to a lockdown party was leaked by ITV News.
Sent by Reynolds, it read: "Hi all, after what has been an incredibly busy period we thought it would be nice to make the most of the lovely weather and have some socially distanced drinks in the No 10 garden this evening."
Martin signed off the email telling everyone to "join us from 6pm and bring your own booze!"
Prime Minister Boris Johnson and his wife Carrie are reported to have among those in attendance at the party alongside 30 to 40 others.
This leak comes just weeks after it was revealed numerous gatherings, including the recently revealed garden party with Johnson in attendance, took place at Downing Street during periods in which the nation was locked down.
