Selected journalists were invited to Number 10 Downing Street yesterday for a briefing from the Prime Minister's chief Europe adviser David Frost on Monday, but correspondents from organisations who were not on Downing Street's hand-picked list also tried to get in.
When political correspondents arrived inside Number 10 they were asked their names and told to stand on opposite sides of the entrance hall - either side of a rug.
Boris Johnson's director of communications Lee Cain - a political appointee - then invited those on one side to enter and told those on the other to leave.
In protest at the treatment of colleagues from rival organisations, all the journalists present chose to walk out rather than receive the briefing.
Batley and Spen MP and Shadow Culture Secretary Tracy Brabin said in the Commons today: "The ability of the lobby to have access to briefings without favour is a longstanding tradition and one that is vital to the health of a functioning democracy."
Ms Brabin questioned who decided which journalists could attend a briefing from Mr Frost, adding: "What was the selection criteria and if that decision was made by a special adviser, are they in violation of the code of conduct of special advisers and the civil service code?"
She said: "The Government's behaviour in these matters threatens the civil service's core values of impartiality and objectivity. It also brings into question the integrity of future Government-media briefings and the conduct of its special advisers and damages a free and vibrant press."
Responding, Cabinet Office Minister Chloe Smith, claimed the Government was "committed to being open in its dealings with the press and to the principles of media freedom".
To laughter from opposition MPs, she added: "The events of yesterday were a very good example of this."
Ms Smith said no journalists are barred from "official briefings hosted by the Prime Minister's spokesman", telling the Commons: "It is entirely standard practice for the Government to host additional technical specialist briefings, as was the case yesterday."
Members of the lobby - accredited journalists who work in Westminster - are free to attend two briefings a day with the Prime Minister’s official spokesman.
But the incident at hand was an additional technical briefing which only select journalists were invited to.
The journalists excluded included outlets viewed as left-wing or critical of the Government, plus the Scottish and regional lobby.
Conservative former Cabinet minister Damian Green said: "There clearly do need to be better arrangements for lobby briefings than were taking place yesterday.
"But I do detect the faint air of fake outrage here."
He added: "What we're seeing here is some fake outrage and a mass outbreak of snowflakery."
SNP MP Pete Wishart said: "Yesterday was a black day for press freedom and no amount of sleekit, self-justified nonsense from the Honourable Lady is going to get her off the Trumpian hook.
"The next thing the Prime Minister will be talking about fake news and banning broadcasters, oh wait, he already has! Just how sinister can it get?"
Ian Murray, executive director of the Society of Editors, said there were "very real" concerns for press freedom.
He said: "The Society of Editors commends the collective action of lobby journalists to walk out of the briefing and all eyes are on No 10 to make a swift turnaround of their decision.
"Yesterday's actions are very much at odds with the pledges made for freedom of expression by the Prime Minister in his Queen's Speech in December."