Proposals to scrap the need for two sources of evidence in Scottish criminal trials have been criticised at the Scottish Tory Party conference.
The plans were passed by three votes at stage one, despite concern from some legal figures and opposition parties.
The Scots Tory chairman Richard Keen said the legislation highlighted "a fundamental problem" with the current constitutional set up in Holyrood.
Supporters of the move say it could increase access to justice.
However, opponents fear it could result in a rise in miscarriages of justice.
The measure was part of the Criminal Justice Bill, approved in principle by 64 votes to five, with 57 abstentions.
The legislation, which brings in a series of reforms to the justice system, still faces two further stages of parliamentary scrutiny before becoming law.
During a debate on justice at the conference in Edinburgh, Mr Keen - who is a QC - said the SNP majority government had appointed a presiding officer, it had an effective majority on the Scottish Parliament's committees, and used its MSPs to "force through legislation".
He said there was no Scottish equivalent of the House of Lords to provide a check on this.
Mr Keen added: "We are faced in my opinion with a very real constitutional issue as a result of this Bill being pushed forward by the SNP government."
Conservative justice spokeswoman Margaret Mitchell said there was an "absence of any checks and balances at Holyrood today".
She said: "The integrity of the parliament has been opened up to intense scrutiny over this issue."
Delegate Ross Sanderson told the conference: "This debate has now moved on from one about policy to one about the integrity of the Scottish Parliament. There is now a legitimate concern that the Scottish Parliament is not adequately equipped to hold a majority government to account."
Scots Law has a unique requirement that accusations be corroborated, meaning there must be two separate sources of evidence before a case can proceed to trial.
It is a centuries old rule, which has been described as "outdated" by justice minister Kenny MacAskill, who believes it stands in the way of too many cases going forward to court when there would be prosecutions in other countries.
Its abolition has been supported by the the Crown Office, Police Scotland and campaigners for victims of domestic violence and rape.
Many criminal lawyers and judges are against the reform, as are the majority of Scottish Parliament's Justice Committee.
During last month's vote on the Bill, the Scottish Conservatives joined Labour, the Lib Dems and Greens in calling on the Scottish government to ditch the proposals while a full review of their impact was carried out.