Business leaders and politicians say a reduced train service between Sheffield and London is already hitting trade and the northern economy.
East Midlands Railway (EMR) cut the number of journeys between Sheffield and London for the foreseeable future, with one businessman calling the route "critical for the survival of some businesses".
EMR said there were "service reliability issues" with a new fleet of trains and the reduced timetable would remain in place while it worked with the manufacturer.
David Richards, co-founder of Yorkshire AI Labs, said it was vital for businesses to have a reliable train service between South Yorkshire and the capital.
"EMR's announcement of a reduction in services and a smaller fleet in the route is pretty disastrous," he told BBC Radio Sheffield.
"This route is absolutely critical for the survival for some businesses, this will definitely impact on our economy.
"I had three investment bankers coming up from London to Sheffield for meetings and they had to travel up the day before.
"We are just asking for trains to be on time so people can get here, do meetings all day and then go home. They should be able to do this in a single day."
He continued: "When investors from London can actually make it up to Sheffield, they see this phenomenal opportunity.
"We have wonderful businesses and there are lots of opportunities here, but people simply have to be able to get here."
Richards, who lives in Sheffield, said he would now make a 45-minute drive to catch a train in Doncaster as the Doncaster to London LNER service was more frequent.
EMR said it had avoided consecutive cancellations and spread service reductions as evenly as possible across Sheffield, London and Nottingham routes.
The temporary timetable would be kept under regular review, it added.
Matt Travis, president of Barnsley and Rotherham Chamber of Commerce, said businesses in Rotherham would also now travel to Doncaster to pick up a London train service.
"EMR's reduced timetable on the Intercity routes is impacting for our local businesses as there is reduced reliability for same-day travel," he said.
"The reduced timetable and cancellations are resulting in lost productivity, increased travel costs and challenges in maintaining face-to-face meetings.
"It is just another hurdle for businesses to cope with following on from a series of challenges over the past few years and is frustrating."
Liberal Democrat peer Shaffaq Mohammed, who lives in Sheffield, said he had recently been impacted by rail disruption while travelling to the House of Lords.
"We have either had cancellations or delays for weeks now," Lord Mohammed said.
"Recently my train set off from Sheffield on time, but the journey was delayed and they said due to ongoing faults it was terminating at Leicester.
"The train was already quite full because EMR have cut a number of services.
"We waited half an hour and then had to join another train which was already full."
He continued: "On another occasion, my colleague Paul Scriven was on a train an hour before me. His train then developed a fault and had to pull over.
"He waited on a platform for 25 minutes to join my train, but it was already jam-packed so we both stood up the whole way."
Lord Mohammed says he had raised the issue with rail minister Lord Peter Hendy.
The Department for Transport said it had asked EMR "to do everything possible" to minimise disruption and make services more reliable.
The delays are due to problems transitioning to a new fleet of trains.
Manufacturer Hitachi Rail said the new fleet had been in passenger service with EMR since December 2025 and more than a third of the fleet had now been delivered.
It said it was working closely with industry partners, implementing improvements and was focused on delivering the remainder of the fleet and supporting EMR as more new trains enter passenger service.
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