Daily briefings for journalists are held by the Number 10 spokesman
Gordon Brown must ensure ministers and their advisers make major policy announcements to Parliament first and not the media, peers have said.
The House of Lords Communications Committee said it found "friendly" journalists were being told the content before it had been formally announced.
This was to secure favourable and prominent coverage in return for exclusivity, the committee said.
The report also recommended televising Downing Street lobby briefings.
It said the current Westminster lobby system, where the prime minister's spokesman give journalists access to government information, was a "barrier to openness".
Broadcasting the sessions would help to "dispel the myths" and the "sense of secrecy" surrounding them, the report said.
Lord Fowler, committee chairman, said it was "vital" that important announcements were first made to Parliament.
"When Gordon Brown became prime minister he said it was his aim to put Parliament back at the centre of political life.
"However his premiership has not ended the trend for ministers and government departments to make their policy announcements outside Parliament first.
"It is important that this is stopped. There should be no question of ministers giving policy decisions in advance to favoured journalists or newspapers.
"Gordon Brown should now remind his ministers of the requirements in the ministerial code."
The committee report also highlighted a 92% rise in the number of special advisers in government departments since 1996, and a 73% rise in the number of press officers over the past 10 years.
A further recommendation was for high-flying civil servants to spend time working in departmental press offices.
The purpose of the report was to look at whether the government had improved communications following the 2004 Phillis Review.
The review came after the controversy over Jo Moore, the transport special adviser who suggested 11 September 2001 was a good day to bury bad news.
The review group said that episode showed there was potential for confusion between the roles of impartial civil servants and politically appointed special advisers.
It argued a culture change from politicians, civil servants and the media was needed to restore trust.
One of the recommendations made was to televise lobby briefings.