The Surrey Economic Partnership said one of four proposals under the Brighton Main Line Route Utilisation Strategy would be bad news for Redhill.
The partnership fears the town could lose fast trains to London Victoria.
A three-month consultation by the Department for Transport (DfT) ended in December and a decision is now due.
The DfT's aims are to "reduce overcrowding, increase route capacity and deliver better performance [between Brighton and London]".
"It will add to journey times and we feel that will make Redhill a less attractive place for people to live and work"
Three of the four options would see the non-stop Gatwick Express service axed.
However, a fourth option - retaining some dedicated Gatwick trains - would "result in a significant degradation of the Redhill-London service".
Nigel Horton-Baker, chief executive of the economic partnership, said: "It could mean putting the trains from Redhill up to London on the slow track, stopping them at intermediary stations all the way.
"It will add to journey times and we feel that will make Redhill a less attractive place for people to live and work."
Mr Horton-Baker said the partnership had written a "strong and detailed letter" objecting to the proposal.
The options for axing the Gatwick Express have also been criticised, with BAA and airport operators campaigning for it to be saved.
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