Southern rail users with disabilities face delayed journeys or the prospect of no longer being able to board some trains after the company said there was no “cast-iron guarantee” that assistance would be available at all stations.

Southern has admitted it may have to book taxis for disabled travellers who cannot complete their journey because the only member of staff on the train is the driver.

Previously there were 33 stations across the Southern rail network where passengers in need of assistance to get on or off the train could turn up and be guaranteed help.

Southern staff have held a series of strikes in recent months in protest at the replacement of conductors with onboard supervisors (OBSs).

Conductors are in charge of opening and closing doors on trains but OBSs are not. Under the old system both driver and conductor must be on board before the train can proceed; under the new system the train can proceed even if the OBS is absent.

Southern, which is owned by Govia Thameslink Railway, has said there will be a net increase of about 130 onboard staff under the new system.

But the right to guaranteed assistance has quietly disappeared. Previous train maps specified the stations where passengers needing assistance could turn up and travel. Now, the maps on the trains say that if such passengers do not book help in advance, “there might be a significant delay to your journey”.

A spokesman for GTR said: “It would be correct to say that there is no cast-iron guarantee that passengers with accessibility requirements can spontaneously board a train in the assumption there would be a second member of staff on board every train.

“On the rare occasion a train does run without an OBS, the process is to alert any customers on the platform (via announcement and use of the screens) to the fact that the next service does not have an OBS and for them to contact the team (help point of free phone number).

“The team will then provide the same guidance and options as a station team member to ensure they can complete their journey – from advice on when the next service with an OBS is due at the station through to book a free taxi for the passenger to continue their journey. This could be to end destination or to the nearest accessible station where boarding assistance can be provided.”

The change could mean significant delays to the journeys of disabled passengers and others needing assistance. And even disabled passengers who book assistance ahead of travelling cannot be guaranteed help.

The Paralympian Anne Wafula Strike, who this month told the Guardian of having had to wet herself on a train journey because there was no accessible toilet working, expressed concern about the changes.

“As disabled people, we also have a life. Some of us commute, have deadlines to meet and meetings to attend,” she said. “I am concerned that it is becoming more and more difficult for disabled people to have the same access to transport systems as able-bodied ones.

“Why should we as disabled people have to wait on a platform for another train because there is no second member of staff on a particular train, and why should we have to wait for a taxi partway through the train journey we have chosen to take?”

This month Sandra Nighy, a wheelchair user, said she had booked assistance to board a Southern train at West Worthing but was left stranded on the platform on 6 January. A GTR spokesman said the company was investigating.

In October the transport select committee said in a report: “We are concerned that no official impact assessment has been made of the potential effects of [driver-only operation] on disabled people’s access to the railway.”

A Department for Transport spokesman said: “We are determined that disabled people have the same access to public transport as non-disabled people, and that is why we have committed over £400m to improve accessibility at stations since 2010.”

Malcolm Chisholm, an OBS with Southern and an RMT union rep, said: “GTR’s modernisation plan offers at most a minimal improvement to reliability but will greatly disadvantage those requiring assistance to board services and will reduce the number of disabled customers using the service. This should be recognised by GTR as an unacceptable price for the introduction of their modernisation plans.”

The GTR spokesman said a team had been established to provide extra support to disabled passengers.

“It will only be in extreme circumstances where we run a train without a second person on board at short notice,” the spokesman added.