Councils, colleges and health boards are not doing enough to cut greenhouse gas emissions in the transport sector, according to a campaign group.
Transform Scotland said 60% of the main public sector bodies had no strategy to reduce environmental costs of travel.
It called on the government to give public bodies one year to "get their house in order".
It praised pockets of good practice, including West Lothian Council and the Scottish Environment Protection Agency.
It also commended the National Library of Scotland for their efforts on travel planning, and called on others to "show such leadership".
Transform Scotland's report Doing their Duty? said public bodies were too dependent on air, rather than rail services, for travel between Scotland and London.
It did, however, note evidence of a move towards the use of low-emission vehicles.
But the campaign group said much more needed to be done to help deliver Scotland's climate change targets.
'Decade of advice'
Transform Scotland chairman Phil Matthews said: "Transport is one of the key sources of greenhouse gases and public bodies have a vital role to play in cutting emissions.
"It is therefore surprising that the wider public sector appears to be taking so little action to help deliver Scotland's legally binding climate change targets.
"We find it particularly dismal that over 60% of public bodies do not have a travel plan in place, despite this being the most effective way to cut emissions from the transport sector, and after a decade or more of free travel planning advice being offered to them."
He said public bodies should be given a year to improve their record, and those failing to take action should be held to account by then Scottish ministers.
Report author Aoife Parker-Hedderman said: "Given that transport is the second-largest source of emissions, and given the scale of the Scottish public sector, significant progress is unlikely to be made in reducing emissions unless there is evidence that public bodies are taking action.
"So this report attempts to fill the gap, presenting a summary of their performance on three key areas of sustainable transport policy: travel planning, low emission vehicles, and travel to London."
A spokesman for local government umbrella body Cosla said councils were committed to Scotland's declaration on Climate Change and reducing emissions.
He added: "We are one of a few countries where all councils have signed voluntarily a challenging declaration to address the implications of emissions reduction across all our services to communities and have publicly reported on it annually for five years with the support of the Sustainable Scotland Network.
"However, member councils accept the need to show leadership and are pleased the report highlights the positive results from joint Scottish and local government sustainable transport initiative Smarter Choices and Smarter Places across Scotland.
"We all need to work together to make the low carbon vision we aspire to Scotland a reality."
He added that all councils were greening their fleets through the procurement of electric vehicles, and council leaders had recently agreed funding for an electric charging network across Scotland, in addition to further funding for low emission vehicle procurement and a range of local incentives to encourage sustainable transport locally.
A Scottish Government spokesman said it encouraged public bodies to show leadership on the climate change agenda through sustainable transport policies.
It also said a Public Sector Climate Leaders forum would meet for the first time next week.