Cleaners left to mop up the mess after lockdown parties in No10 were treated “appallingly”, unions said on Wednesday.
The senior civil servant added that “this was unacceptable”.
On December 19 one cleaner who attended a room in Downing Street following a Christmas party found “red wine spilled on one wall and on a number of boxes of photocopier paper”.
That same day Boris Johnson announced London was being put under Tier 4 Covid restrictions because of the speed at which the virus was spreading in the capital.
Dr Paul Chiy, founder of the Cleaners Union, said they were “appalled that underpaid and overworked cleaners” were left to mop up the mess after parties.
“These are the same cleaners that were at the frontline of the pandemic and were called 'key-staff',” he said.
“This is immediately forgotten away from the camera. They are not only left to clean up the dirt but are treated as such. This is unacceptable.”
The Cleaners and Allied Independent Workers Union said they were “deeply concerned” by Ms Gray’s findings.
“We are saddened to see that there were multiple examples of a lack of respect towards and poor treatment of security and cleaning staff,” a spokesman said.
“Cleaners and security staff were on the front line throughout the pandemic and continue to provide a vital role within society in keeping the public safe and well. They deserve our sincere gratitude and no less.”
Jim Melvin, chairman of the British Cleaning Council, said cleaners appear to have been treated with “contempt”.
He added: “Cleaning staff are hardworking, professional and deserve to be respected in their vital work, just like anyone one else, and certainly how the people concerned would expect to be treated.”
Labour deputy leader Angela Rayner accused Mr Johnson of turning Downing Street into a vomit-stained Bullingdon Club,”.
“The treatment and mockery doled out to cleaning and security staff that has now been exposed is beyond shameful,” she said.
At a Downing Street press conference, Mr Johnson said: “On the behaviour of staff and alleged rudeness, I don't know who is specifically guilty of that. "In the course of my apologies today, I began to make some inquiries and I will, of course, continue them.
“As I said earlier on, I think it is utterly intolerable for people to be rude to hard-working staff. "At the very least, they should apologise but I don't yet have the names of those who were responsible.”