Up to 4,000 of the 77,000 young people who left school this summer will not find an apprenticeship, according to the Swiss Federation of Trade Unions.
However, the government said there were enough training places available - just not in all professions.
Speaking at a news conference on Wednesday, senior trade union official Peter Sigerist said the number of training places on offer had to increase by between 15 and 20 per cent to cover demand.
He added that the situation was critical and that a fifth of this year's school-leavers would have to take a gap year, something which Sigerist said should be avoided at all costs.
"These young people will end up in a long queue with no guaranteed prospects," he warned.
In July, youth unemployment rose by 4.7 per cent, according to latest figures by the State Secretariat for Economic Affairs.
But Sigerist stressed that the actual numbers were larger, as half of those affected did not register at their local unemployment office.
While the number of apprenticeships has increased in recent years, the number of school-leavers has also gone up.
To remedy the situation, Sigerist called on the authorities to dip into their pockets to fund alternatives such as apprenticeship workshops or courses at vocational schools.
He said such measures could be introduced in the short term but the political impulse necessary to push them through was lacking.
He added that firms which were unable to offer a full apprenticeship could at least offer a foundation year.
However, the federal office in charge of professional training has dismissed the perceived dearth of opportunities.
"There are enough apprenticeships but they are not in the areas that are in demand. For example, there is a surplus of vacancies in the construction, butcher and catering trades," spokesman Jacques Filippi told swissinfo.
He said the federal authorities were doing all they could to encourage the provision of training opportunities. But he added that it was up to the cantons to create extra places.
"We spent SFr21 million on measures to improve the conditions surrounding the market for apprenticeships. We introduced the vignette a year ago, which is given to each firm that trains up apprentices. This improves their image," Filippi said.
For its part, the Swiss Employers' Association said about 95 per cent of all school-leavers went into an apprenticeship.
It said there were many vacancies across Switzerland, but certain jobs were unpopular.
"The construction industry has a bad image. The unions are calling for the retirement age for this sector to be reduced to 60, saying that people die earlier if they are active in this trade," said spokesman Urs Meyer.
The Swiss Federation of Trade Unions has published a brochure to help apprentices know their rights, which takes into account the new law on vocational training.
The guide replies to questions apprentices might have on contracts, holiday entitlement, salaries, harassment and so on.
The unions hope that the publication will encourage young people to defend their rights.
Average monthly wages for apprentices for first and fourth years of service in canton Zurich:
- Car mechanic SFr600/1300
- Hairdresser SFr400/700
- Electronic technician SFr450/1000
- Gardener SFr450/900 (lasts three years)
- Commercial clerkship SFr670/1360
- Road worker SFr1180/2100 (lasts three years)