Helen Morgan, Liberal Democrat MP for North Shropshire, said the long-awaited report from Sue Gray made it "clear he thinks he took the public for fools".
Mrs Morgan, who was elected last December on the back of a Tory lobbying scandal – and the fallout from attempts to avoid punishment for the matter – said the report again highlighted the contrast with members of the public who made numerous sacrifices as part of the pandemic restrictions.
Mrs Gray's report concluded that the "senior leadership" in Boris Johnson's Government must "bear responsibility" for the culture which led to coronavirus lockdown rules being broken.
Mrs Gray's report said the public would be "dismayed" by a series of breaches of coronavirus rules in No 10 and Westminster.
"The events that I investigated were attended by leaders in government. Many of these events should not have been allowed to happen," she said.
The Metropolitan Police has issued 126 fines for rule breaches in No 10 and Whitehall, with the Prime Minister receiving a single fixed-penalty notice for his birthday party.
But senior civil servant Ms Gray condemned the wider culture that had been allowed to develop under Mr Johnson's leadership.
Mrs Morgan said the report outlined "disgraceful behaviour" at the heart of government.
She said: “Sue Gray’s report makes it clear that Boris Johnson drank and partied while people across the country were making sacrifice after sacrifice.
“Families were heartbreakingly missing funerals, births and marriages while their Prime Minister was flagrantly breaking the very rules he was setting.
“Our dedicated nurses, doctors and paramedics are still dealing with pressure never seen before in Shropshire and now learn that Boris Johnson’s Downing Street has been the scene of raucous parties and disgraceful behaviour.
“My heart goes out to everyone who suffered while believing they were doing the right thing by staying at home alone.
“It is time Johnson did the decent thing and resigned. We have all seen the pictures. It’s clear he thinks he took the public for fools.”