A long-standing drinking culture in Downing Street saw staff start boozing at lunch and waking up there in the same clothes the next day, a former No 10 official has said.
Sonia Khan discussed the historic drinking culture at the heart of Government as Boris Johnson considers demanding an alcohol curb as he battles allegations of lockdown-breaching parties.
The Prime Minister will reportedly order a booze ban in No 10 under the so-called 'Operation Red Meat' policies designed to save his leadership from demands for his resignation.
Ms Khan, who worked in No 10 and the Treasury during the leaderships of former prime ministers David Cameron and Theresa May, said drinks had long been "normalised" in Downing Street.
But she said historic use was "totally different" to the allegations of partying while the public was ordered to abide by restrictions to tackle coronavirus.
"Usually these drinking sessions are sandwiched between pieces of work, so it feels like a very, very routine thing," Ms Khan told BBC Radio 4's World At One programme.
"Drinks could start at lunch time, they could start a little bit later in the day - different teams do things very differently - but the idea of mini fridges or having drinks underneath your table wasn't uncommon."
Ms Khan said "senior people at No 10" going back two decades had used drinks as a way of thanking staff for working "very, very long hours".
Asked if people had been so hungover they had slept on sofas in Downing Street, she said: "I did see a few instances of that - people waking up in the same clothes the next day.
"But obviously I didn't work during a pandemic and it didn't happen very often back then, I should say. I can't speak for what it's like now."
Mr Johnson was forced to apologise after it emerged his principal private secretary, Martin Reynolds, invited more than 100 members of staff to a "bring your own booze" party in the No 10 garden in May 2020 during the first lockdown.
The Prime Minister admitted he attended but argued he believed it was a work event that could "technically" have been within the rules.
Ms Khan said: "Drinks in No 10 ... feel like such a normalised thing so it doesn't feel like anything out of the ordinary.
"Now in a pandemic that's totally different and you can absolutely say that people should've had the oversight, given that they are all so smart and intelligent."
Ms Khan worked as a civil servant in No 10 under Mr Cameron, before working in the Treasury as a special adviser during Mrs May's leadership.
She briefly remained in the Treasury after Mr Johnson took over, but was marched out of Downing Street by armed police after being sacked by Dominic Cummings in August 2019 over allegations of leaking.
She later settled a claim against the Government for unfair dismissal.
Over the weekend, Conservative Party chairman Oliver Dowden said there are plans to "address the kind of culture that has allowed" alleged rule breaches.
This will include a crackdown on drinking in Downing Street, The Sunday Times reported.
On Monday, the Prime Minister's official spokesman said Mr Johnson will set out his changes after senior civil servant Sue Gray publishes her inquiry into allegations of partying.
He told reporters: "I think I'd point back to what the Prime Minister's talked about, that there's a shared recognition that No 10 should be held to the highest standard.
"There are things clearly we didn't get right and we need to take responsibility for those. But beyond that, I think it's right to let the report be published and then you'll hear more from the Prime Minister."
