The Confederation of Passenger Transport (CPT), which represents bus operators, is also worried that youngsters without cards being told by bus drivers they still had to pay would create a “negative first experience” for such new passengers.
The initiative could prove hugely significant to the fortunes of the struggling bus sector as some 930,000 people up to the age of 21 will be eligible for free travel across Scotland, but they need to update their existing cards to benefit.
However, Transport Scotland has put a planned “high-profile marketing campaign” to publicise the scheme on hold because of the Covid pandemic and only about 65,000 new cards are believed to have been issued.
It has urged only those with an “essential need to travel”, such as for education or work, to apply for the new versions of the Young Scot or national entitlement smartcard required for free travel.
CPT said the decision could lead to confusion.
Scotland director Paul White said: “Our understanding is that the majority of eligible young people will not have valid cards by the launch date.
"Whether this is because they do not intend to use the scheme, or they are unaware of the need to apply for a card, is unclear.
"Our concern is that young people attempt to use the scheme on Monday unaware that if they do not have a valid card they will be required to pay a fare.
"It is unfair on our front line staff to be placed in the position of having to inform young people of the scheme rules as they board.
"It may lead to increased boarding and journey times for other passengers and, perhaps most importantly, it will create a negative first experience of the scheme for the young people we wish to make regular bus users.
"The bus sector will be doing all we can to make the launch of the scheme a success and mitigate these potential issues."
The fears were shared by companies such as Borders Buses and West Coast Motors, which operates services in Argyll and Citybus in Glasgow.
It said: “Our concern from Monday is some U22s boarding without a valid card and expecting free travel.
"Unfortunately, those individuals will have to pay their fare as normal.”
Another major operator said: "No-one wants to see bus drivers put in an impossible position faced with young people without the right card to travel for free, or journeys being delayed."
Transport Scotland said most under 22s knew they needed a new card.
Its spokesperson said: “We will continue to inform young people of the steps they need to take to use the scheme and know the vast majority will realise they cannot get their travel free until they have an entitlement card.
“We know CPT welcomes the beginning of free bus travel for under 22s as it has huge potential to support the recovery of our bus services from the pandemic and help create a future generation of bus users – which is good for bus operators, our communities and our planet.”