Plaid Cymru MPs say two lawyers have advised them that an attempt to impeach Tony Blair over the Iraq war would be a legal procedure.
The legal judgement was published on Thursday
The party made the announcement its members gathered for the first day of its annual conference in Llandudno.
Plaid says more than 20 MPs have backed the campaign to wield a power last used in 1806 over the prime minister's reasons for going to war.
Labour critics have dismissed it as a silly stunt.
The legal judgement was published on Thursday by two senior lawyers who share the same chambers - Matrix - as the prime minister's wife, Cherie Booth.
Rabinder Singh QC and Professor Conor Gearty said that in the eyes of constitutional law and parliamentary process, Mr Blair could still be impeached.
The case against the prime minister would be that in making the case against the war against Iraq he was guilty of a serious breach of constitutional principles.
Carmarthen East and Dinefwr MP Adam Price, who has been leading the campaign, said: "We have continuously maintained that the prime minister has a case
to answer over his conduct in relation to the invasion of Iraq, and now we have
the legal opinion to prove we were correct.
"No longer can New Labour say that impeachment is obsolete or can they say
that the questions have already been answered.
"As (United Nations Secretary-General) Kofi Annan stated last week, the war in Iraq was both illegal and
damaging," he added.
Donald Anderson, Labour chairman of the Commons foreign affairs committee and Swansea East MP, has previously described it as a "political stunt" and a "no-hoper," while former Labour minister Keith Vaz called it a "silly story for the end of the silly season."