The official review of fire safety regulations launched in response to the Grenfell Tower disaster has been heavily criticised by safety experts.
The review has been accused of having a lack of transparency and alleged conflicts of interest.
A meeting organised by the All Party Parliamentary Fire Safety and Rescue Group in the House of Lords was told it has been shut out of Dame Judith Hackitt's Independent Review of Building Regulations and Fire Safety.
The group secretary, Ronnie King, told the meeting: "We bid for six places on the working groups, of which some have 12 to 14 people on them, but the places were all taken."
Mr King said the official group of MPs and peers, which has been advocating for fire safety for almost two decades, was told: "Sorry, we decline to give you a place."
The Hackitt Review was set up by the Government after 71 people died in Grenfell Tower in June 2017, when fire spread up the outside of the building which had been wrapped in combustible plastic foam insulation and plastic filled cladding panels.
Its six working groups have been given until the first week of March to provide interim answers to a range of questions, but their composition, agendas and minutes are not being made public.
A document sent to all members of the groups, seen by Sky News, states: "Discussions will be treated as confidential, and working groups should exercise discretion in how they share information outside the group."
The review was also criticised for handing a senior role to advisory group BRE, which assured ministers before the Grenfell Tower disaster that the building regulations were "adequate" to cope with combustible plastics on tower blocks.
Debbie Smith, a managing director in the BRE group of companies, has been appointed chair of the review's "materials, systems and product testing" working group which meets at BRE's headquarters.
BRE created the official fire test and assessment process for combustible cladding systems and is paid by plastic materials manufacturers to test their products.
Ms Smith's committee has been asked by Dame Judith to answer the question: "How can we ensure that all materials, products and systems are safe for the purpose for which they are used in complex/high-rise buildings?"
In a statement, a spokesman for Dame Judith said the question was not "directed by BRE".
The statement said: "The working groups have representatives from a wide range of organisations and experts in their field [and] members of the working groups have been encouraged to engage with a wide range of organisations."
Fire safety expert Arnold Tarling said after the meeting that BRE should not be involved in the review.
"They should be excluded for conflict of interest, their paw prints are over everything on this," he said.
"It shouldn't be being done by the advisers who've advised all the way along and those who are friends of the advisers, it should be done by people who are totally independent," he said.
Non-combustible insulation manufacturer Rockwool, who sponsored the meeting in the House of Lords, also called for BRE to be excluded from the review process.
Rockwool senior vice president Gilles Maria was one of several experts at the meeting to criticise BRE's fire tests, accusing them of not exposing combustible cladding to the ferocity of a real building fire.
"The big debate today is whether the [fire] tests should be banned," he said after the meeting.
"If that debate is on the table we should not ask the person in charge of the test to chair the group for that."
BRE told Sky News it was pleased to support the Hackitt Review and said the working group it chairs had no concerns at its first meeting last week.
BRE said its role was "to facilitate an open and expert discussion" and that "the meeting started with us asking all panel participants [...] if they had any issue with BRE's chairmanship; everyone responded positively and unanimously to BRE chairing the working group".
The chair of the House of Lords meeting, Hammersmith MP Andy Slaughter, called for greater transparency.
"We have to have a much clearer remit from Government about what they're trying to achieve, or we have to go in a fail-safe way and say we're only going to use non-combustible materials," he said.
The meeting was also told that the Metropolitan Police criminal inquiry into the Grenfell Tower fire has copies of all correspondence between the Parliamentary fire safety group and ministers.
Mr King said he had handed over "minutes of meetings and correspondence with ministers" to both the police investigation and Sir Martin Moore-Bick's public inquiry into the disaster.
The public inquiry has been told the police are considering charges of manslaughter, corporate manslaughter, misconduct in public office and breaches of fire safety legislation.