The "abysmal service" of a private rail company means it should lose its franchise, a passenger watchdog says.
Customer satisfaction for FGW has dropped to 53% from 66%
London TravelWatch is to write to the government after complaints about First Great Western's (FGW) late trains, overcrowding and poor customer care.
In January FGW apologised to rail users "for not meeting expectations" after a protest by hundreds of passengers.
The company said it was working hard to improve the service, but explained it was not breaching franchise conditions.
London TravelWatch's chairman Brian Cooke said: "Passengers have been telling us just how bad FGW's recent performance has been, and that the standards of customer care and information given to passengers have been appalling.
"This franchise is only 15 months old, but First was running the service before that, so there is no excuse for the abysmal service FGW are giving their passengers."
Mr Cooke said the most recent peak-time performance of FGW trains in the London and the Thames Valley section showed the number of trains arriving on time had fallen to 68.4% and the average over the last year was 75.6%.
This was against FGW's London and Thames Valley target of 92%, he added.
He also claimed the train operator exceeds the target for carrying passengers in excess of capacity, the rail industry's measure of overcrowding.
The London TravelWatch board added that a recent national passenger survey, satisfaction with the service from FGW passengers in London and the Thames Valley, had dropped to 53% - from 66% a year ago.
This compares with an against an average satisfaction rating in London and south-east England of 77%.
A spokesman for FGW said: "The Department for Transport reviews our performance every four weeks and we are not in breach of franchise conditions.
"However, we are aware that our performance has been poor and we have already apologised for this.
"We are working hard to improve our service and we are delivering a comprehensive service recovery plan that has already provided additional and more reliable trains, more staff, a range of timetable enhancements, and improved customer service."