The licensing system was launched after the Morecambe Bay disaster

Unions are concerned the regulations, due to come into force next year, will exclude groups of workers, including those in food processing and packaging.

Any limit on the licensing system would create a "loophole ripe for exploitation", the TUC said.

The union was responding to a government consultation on the plan.

Leader Brendan Barber called on ministers to make sure the legislation was robust and able to deter "ruthless individuals".

The Transport and General Workers Union and the GMB have also appealed for tougher regulations.

'Negative connotations'

A report on the activities of gangmasters will be given to ministers next week, the Department for Environment, Food and Rural Affairs (Defra) said.

A decision on what action to take will be made before the summer recess.

Earlier this year, employment agencies told BBC News they worried about being "tarnished with the same brush" as rogue gangmasters.

Labour providers also complained that the name of the new Gangmasters Licensing Authority (GLA), which will regulate them, had "negative connotations".