An official investigation is being demanded amid concerns that Scottish Government ministers used taxpayer-funded limos to travel from SNP functions.
Nicola Sturgeon journeyed in an official vehicle to and from an election campaign event in Stirling in April. The First Minister was photographed posing on an independence-branded motorbike at the event.
Eight Cabinet ministers also used taxpayer-funded vehicles to leave the SNP general election manifesto launch in Perth in May.
The Scottish Government has insisted the ministers were being picked up to be taken to official engagements.
Labour MSP James Kelly has now written to Ms Sturgeon demanding she launch an investigation to see if the ministerial code has been breached.
“These are serious accusations that must be immediately investigated,” Mr Kelly said.
“The ministerial code is a vital part of our democracy and is essential to retaining public trust in politicians.
“The public and parliament deserves to know how these trips came to take place, what the justification for them is and whether such journeys will happen again.
“Anything less than a full investigation would be a breach of trust and leave SNP ministers looking even further out of touch than they do already.”
Ministers must not use public resources for party political purposes.
Conservative MSP Miles Briggs said: “The public will be deeply uncomfortable at the idea of eight ministers swanning out of such a party political event and straight into a taxpayer-funded car.”
He added: “For half the Cabinet and the First Minister to be involved reeks of privileged complacency. SNP ministers seem to have forgotten that these cars are paid for by the public, and they risk not being taken seriously when they complain about limited resources available.”
A Scottish Government spokeswoman said: “The government car service is available for the use of ministers and Cabinet Secretaries in travel to and from government business. On this occasion, ministers and Cabinet Secretaries were picked up and travelled to a variety of ministerial engagements.
“We continually look for ways to minimise the use of cars for official journeys, for example, by car sharing as happened on this occasion.
“Where practical to do so, ministers also use public transport or walk to their engagements. This is not always practical if it is necessary for them to carry out sensitive government business during their journey.”