The complaint is part of the LTUC's response to a Strategic Rail Authority (SRA) consultation on the Brighton rail route. This proposes integrating the dedicated airport service with Sussex coastal services to boost capacity for commuters.
The committee claimed that crowding on coastal trains and underuse of Gatwick Express trains have been exaggerated in the SRA strategy. It said that travelling conditions would be unacceptable due to luggage obstruction.
It contended that the route would make less money because it would not be realistic to charge a premium fare when many express service features would be lost.
The LTUC response is another blow to the SRA. It follows a campaign launched by Gatwick owner BAA last week against the plans, which it claimed would mean a million extra cars on the road. It also questioned the safety of people waiting on overcrowded platforms.
But an SRA spokesman argued that the changes would benefit passengers because the new service would see trains running every ten minutes instead of every quarter of an hour.
He added that more people would use the route, as trains would be running at only 80 per cent capacity rather than the 130 per cent currently experienced on commuter trains.