First Minister Alex Salmond has been accused of selling access to his office to raise funds for the SNP.
It emerged last night that a meal for four with Mr Salmond at Holyrood was auctioned for £9,000 at a party fundraising event on Tuesday.
A similar lunch with Deputy First Minister and Health Secretary Nicola Sturgeon was sold for £2,000.
Scottish Parliament policy says Holyrood resources must not be used for “any significant party political purposes”.
The SNP insisted that parties have used lunches with politicians as a way of fundraising for “many years” and are “not contrary to any current parliamentary rules”.
In 2007 Tory leader David Cameron was rebuked by the parliamentary standards watchdog for using his tax-payer-funded Commons office to raise money.
The Commons committee on standards and privileges said at the time that Mr Cameron had been “ill advised” to offer meetings at his office for members of a Conservative fundraising club.
Prospective members of the Leader’s Group were promised lunch in Mr Cameron’s parliamentary office after prime minister’s questions in return for a £50,000 subscription fee.
Liberal Democrat chief whip Mike Rumbles said: “Scottish ministers should not be available to the highest bidder.
“I’d have thought that Alex Salmond and Nicola Sturgeon would have learned from David Cameron’s cash for access scandal.”
Labour leader Ian Gray said: “These are very serious allegations accusing Alex Salmond of selling the office of first minister for SNP funds. The implication of cash for access to the first minister and his deputy has to be answered.
“Was it wrong of the first minister to auction lunch with himself in Holyrood to raise £9,000 for the SNP? Such grubby behaviour demeans the office.”
The Scottish Parliament Corporate Body has launched an investigation.
A spokesman said the policy on the use of parliamentary resources states that they are provided to support members with their duties.
He added: “These resources, which include the members’ restaurant, must not be used for any other purpose, including any significant party political purposes.”
The regulations state that parliamentary resources, such as office equipment e-mails, IT equipment, meeting rooms and other resources were for the use of MSPs and not to be used for any other purpose.
The SNP said that could exclude charity activities, and raised the question whether donors were entitled, like anyone else, to be entertained or taken around the parliament.
Last night Mr Salmond, as MSP for Gordon, wrote to Holyrood chief executive Paul Grice seeking clarification of the rules.
Referring to the regulations, Mr Salmond said: “My view would be that this does not include the dining facilities that are paid for as opposed to parliament resources which are provided.”