The government has insisted it is "committed to being open" with the press amid a row over the official system for briefing journalists.
A group of leading reporters decided not to attend a Downing Street briefing on Monday when other journalists - who hadn't been invited - were turned away.
Labour said this threatened democracy, while the SNP said it was "sinister".
But Cabinet Office minister Chloe Smith said it was "routine" for select titles to be given "additional" briefings.
Responding to an urgent question in the Commons, she said there was a clear distinction between the twice-daily briefings by the PM's official spokesman open to all accredited members of the parliamentary lobby and extra briefings given by special advisers, political appointees handpicked by ministers.
Journalists from the BBC and ITV were among those to walk out of a briefing due to be given by the PM's chief Brexit adviser David Frost after No 10 advisers reportedly said publications such as the Mirror, the i and Huffington Post were not welcome.
Raising the issue in Parliament, shadow culture secretary Tracy Brabin suggested it was "not an isolated incident" and there was a trend for the government to decide who had access to information.
She asked who had drawn up the criteria for inviting journalists, whether civil servants had been present at the time, and whether those responsible had violated the code of conduct for special advisers.
"The ability of the lobby to have access to briefings without favour is a longstanding tradition and one which is vital to the health of a functioning democracy," she said.
"Yesterday certain publications were barred from a briefing on future trade talks.
"The code of conduct for special advisers' states that special advisers must not ask civil servants to do anything which is inconsistent with their obligations under the civil service code.
"If that decision was made by a special adviser, are they in violation of the code for special adviser and the civil service code."
In response, Ms Smith said there was nothing unusual in what had gone on and the government strongly supported a free and open press.
"No journalists are barred from official media briefings hosted by the prime minister's official spokesman. It is entirely standard practice for the government to host additional specialist or technical briefings as was the case yesterday."
She said the planned session with Mr Frost was designed to "increase understanding" of the UK's stance in the next stage of Brexit talks.
She urged Labour "to look in the mirror", suggesting the opposition wanted to introduce a Soviet-style licensing of the newspaper industry.
Conservative MP Damian Green said while "better arrangements" were needed for such lobby briefings he detected "the faint air of fake outrage", while his colleague Sir Peter Bottomley said under the last Labour government certain editors had been banned from briefings.