Changes at the Vehicle and Operator Services Agency (Vosa) have resulted in the removal of on-street wardens whose reports were used as the basis for enforcement action against bus firms whose services do not run or are repeatedly late.
The situation has prompted concerns from Scotland's Traffic Commissioner, who regulates bus services, and Green Party MSP Patrick Harvie.
In her annual report, Joan Aitken, the Traffic Commissioner for Scotland, said: "There no longer are Vosa bus compliance officers, their duties now being undertaken in different manner by traffic examiners and the emphasis removed from on-street monitoring. The on-street monitor stands exactly in the place of the travelling public."
A more blunt assessment was given by Mr Harvie, who said: "Bus services in many parts of Scotland are in a shocking state, but it seems that it's only the long-suffering passengers who are there on the ground to see the true situation. Even in areas where local arrangements are in place, many people justifiably regard timetables as a joke."
The Scottish Government said it may introduce bus wardens to fill the gap left by Vosa.
It said a number of transport authorities, including Strathclyde Partnership for Transport, carry out their own checks to ensure bus services run on time. However, it is thought other parts of the country do not have any means of addressing poor performance.
Government agency Transport Scotland said it was aiming to implement changes to bus monitoring next April.
A spokesman for the Confederation of Passenger Transport, which represents bus and coach firms, welcomed Transport Scotland's initiative.
He said: "The bus industry often meets or exceeds a reliability target of 95%. It does not appear that Vosa's revised strategy in Scotland has negatively impacted on reliability or punctuality figures in any way."
Vosa defended the changes, which it said were in line with those implemented in 2011 in England and Wales.
A spokeswoman said: "That moved Vosa's role away from being one of having a small number of dedicated officers observing bus services to build up evidence on punctuality issues, to our trained enforcement examiners in the first instance working with bus operators and local authorities to establish improvement plans (or similar) where apparent failings in punctuality have been identified.
"Such punctuality issues would generally initially be identified by customer complaints that the Traffic Commissioner or Vosa have received."