Transport campaigners have warned Britain is heading towards a rail overcrowding crisis after a “train from hell” got so crowded that police had to help passengers leave it.

With passengers describing conditions on the 10am Paddington to Penzance service as “overcrowded”, “shambolic” and “unsafe”, Great Western Railway (GWR) staff asked about 100 people boarding at Plymouth to return to the platform.

British Transport Police assisted GWR staff in getting the passengers back on to the Plymouth platform before the train could continue to Penzance.

As the train made the five and a half hour journey from London to Cornwall on Good Friday, travellers posted pictures on social media of people crammed into carriages and sitting or lying in the aisle between seats.

In a series of tweets, Nigel Chapman, the former director of BBC World Service, said: “On train from hell. Shambolic GWR train for Penzance. Overcrowded and no management by stroppy staff. Dangerous and unsafe. Pregnant woman fighting to get to loo.”

It prompted Stephen Joseph, the chief executive of the Campaign for Better Transport to tell The Independent: “This is symptomatic of the fact we are heading towards a crisis. There are many trains all over the country where people are packed in like sardines.

“It reflects long term underinvestment in the railways, at a time when people are in some cases being charged very high fares. We need to see more of a strategy and investment for tackling this – otherwise we will be facing a huge crisis, and the overcrowding we have seen here will become general.”

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He added: “There are plans to electrify and upgrade that [London-Penzance] line, but frankly they can’t come soon enough.”

A GWR spokesman said passengers who were told to get off the two-carriage stopping service at Plymouth were subsequently able to board a high speed eight-carriage train.

He said: “To improve comfort and safety we asked some customers to use a slightly later train which had been put on to cope with additional passengers.

“One can expect trains to be busy during the busiest travelling times. We provided as many services as we were able with the rolling stock available to us.”

He added: “We wouldn’t operate a service that we deemed unsafe. We take the safety of our services and our passengers very, very seriously, which is why we didn’t continue that train with the numbers of people on it [at Plymouth].”