Cyclists taking part in the annual London to Brighton bike ride will not be able to get the train home this year.
The event started in 1976 when 30 friends rode from Hyde Park
A lack of room on new high-tech rolling stock means this year, for the first time, participants will not be able to return to London by train.
About 27,000 people take part in the charity ride each year, with many of them traditionally returning to the capital by train.
Last year South Central Trains ran 30 special services, with seats removed from old carriages, but it has said it cannot do the same this year.
The event, due to take place on 20 June, runs over a 56-mile route from Clapham Common to Madeira Drive on Brighton seafront.
Since 1980 it has been organised by the British Heart Foundation (BHF) and last year's event raised almost £2.5m.
In 2003 about 5,000 cyclists took advantage of the special trains run by South Central.
But the train firm is in the process of introducing £856m worth of new rolling stock and is scrapping its old trains.
A spokesman said the new trains have room for only two or three bikes per carriage and cannot have the seats removed to make room for more.
He said South Central had informed the BHF of the situation in 2002, to give it enough time to make alternative arrangements.
The new trains have no guards' carriage where bikes can be stored
The spokesman said: "In the past we were able to work with the British Heart Foundation to provide services, but unfortunately we are no longer in a position to do it.
"They have known for some time that this would eventually happen - there is no ill feeling."
Julie Sorrell, head of events for the BHF, said: "We were provided with plenty of advance notice from South Central Trains and have made alternative travel arrangements - which will include a coach and lorry service."
But the CTC, the UK's national cyclists' organisation, has been more critical of the situation, claiming the change reflects the railway industry's attitude towards cyclists.
Director Kevin Mayne said rail operators were tailoring their services to the needs of commuters when trains were also used by many other groups of people.
He said the reduction in space which meant less room for bikes also affected wheelchair users and parents with children in pushchairs.