The TUC is demanding a Low Pay Commission probe into the employment of young workers on unpaid internships.
Interns often work unpaid in the hope of getting a job
The TUC says it does not want to block genuine work experience but it fears the system could lead to exploitation.
It follows concerns MPs' offices may have broken minimum wage legislation by advertising for interns to work unpaid.
And evidence to the commission shows many firms - especially in the media - are demanding long periods of unpaid work as a price of entry to the sector.
This could be regarded as exploitation rather than opportunity, the TUC said.
The adverts placed by MPs staff for interns required candidates to be available five days a week and have good office skills but did not offer any pay.
The House of Commons' guidelines on the use of graduates are now being rewritten.
BBC labour affairs correspondent Stephen Cape said the matter was a "grey area of law".