The introduction of the new Greater Anglia Intercity trains has meant that the company is no longer able to support the Dunwich Dynamo cycle.
Greater Anglia has supported the event, which is due to take place at the end of July, in the past.
The Dynamo sees 2,000 cyclists ride through the night from London to Dunwich beach. Many have used Greater Anglia trains to take themselves and their cycles back to the capital.
However the new Stadler-built Intercity trains do not have as much room for cycles as the old traditional locomotive and carriages which included a guard's van with space for cycles.
The new intercity trains are longer than the 40-year-old models they replaced and have more seats, with a dedicated space where cyclists can sit close to their bikes.
That means Greater Anglia is not able to support the event this year - and is urging cyclists to find another way to return to London.
There are six cycle spaces on each of Greater Anglia’s new intercity trains – but this is not enough for the large number of cyclists who take part in the event, which takes place on July 24 and 25.
Kim Bucknell from Greater Anglia, said: “Unfortunately we are unable to support the Dunwich Dynamo this year and we are unable to offer mass transit for the large number of cyclists involved
“Our new Intercity trains have six cycle spaces each and there is no limit on the number of folding bikes we can carry, but this is not sufficient for the number of event participants.
“We are advising cyclists to find an alternative way to transport their bikes back to London. We’re sorry not to be able to help participants, but these restrictions are in place for the safety of our passengers.”
A spokeswoman for Greater Anglia said: "We have better provision for cycles on our new trains - usually there is plenty of space and people can sit near their bikes. But we just don't have the capacity to carry the participants in a major event like the Dunwich Dynamo."
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