The number of seats on the Metropolitan Line is to be dramatically reduced for three years.
New trains are to be introduced on the line during 2010 but will have less seating, forcing passengers to endure long journeys on their feet.
Commuters were promised by London Underground that the reduction would be compensated by a more frequent service - but this has now been ruled out because the signalling system will not be upgraded until 2013.
Over the past few months, representatives from transport groups have been urging London Underground to re-think its plans, but they have been unsatisfied with its response.
Regular train users are particularly worried about the impact on elderly residents, pregnant women and young children, who could be forced to stand for up to an hour on the already crowded trains.
Marc Thivessen, 23, of Hindes Road, Harrow, uses the Metropolitan Line every day to get to work in central London. He said: "The whole Underground is a joke. It is pretty difficult already to get a seat during rush hour but it is going to get worse.
"I am young and can squeeze onto a train at the moment but the changes are going to affect the elderly, disabled and those who are less able to stand. It basically means the most vulnerable people in the borough will find it difficult to travel during rush hour."
Anthony Wood, chairman of Harrow Public Transport Users' Association, is furious the signalling upgrade will take three years to complete after the new trains are introduced. He said: "When we were consulted on the new trains for the Metropolitan Line we were told that by increasing the number of trains per hour the number of seats per hour would be kept the same as now.
"Over the past few months we have been making representations to London Underground and the Mayor of London, Boris Johnson, about this unsatisfactory result. They have sympathised but can do nothing about the situation that will face passengers."
London Underground maintains the introduction of a standard train is necessary so they can be used on all lines across the capital and will, in the long-term, offer a more efficient service.
A spokeswoman from Transport for London said: "A consequence of Metronet going into administration was that a decision was taken to retender the contract to deliver a new signalling system, which includes the Metropolitan Line.
"At present, tender documents are being developed to ensure the new system delivers the necessary improvements. In the interim London Underground is looking at ways to increase the frequency of services when the new trains are introduced."