A group of MPs is calling for contracts awarded to a private hospital patient transfer firm to be terminated.
A debate in Parliament on Tuesday heard Thames Ambulance Service were routinely leaving elderly and vulnerable patients waiting for hours.
Andrew Percy, MP for Brigg and Goole, said in one case an 82-year-old patient was left waiting in a hospital doorway in snowy conditions for three hours.
The firm acknowledged there were issues but said it was "making progress".
North Lincolnshire Clinical Commissioning Group, which covers Mr Percy's constituency, awarded its contract to Thames Ambulance Service in April 2016.
The firm has since expanded its operations to Hull, Lincolnshire and Leicestershire.
Mr Percy described the service in North Lincolnshire as "appalling".
"Going to hospital is stressful enough, but especially if you don't know if you are going to get home at the end of your treatment, or appointment," he said.
"There is something seriously failing here. I have no confidence this service will improve and think it is time this contract was terminated."
Mr Percy has called for the government to intervene.
Scunthorpe MP Nic Dakin, Hull MP Diana Johnson and Great Grimsby MP Melanie Onn also spoke about problems with the service during the debate.
In response, junior health minister Caroline Dinenage said: "Every single one of these incidents is totally unacceptable, and in many cases, very distressing."
However, she said: "We have to allow the clinical commissioning group to take the necessary steps to ensure the service is improved."
She said a "robust recovery plan" had now been put in place in North Lincolnshire.
In a written response, Thames Ambulance Service said: "We are improving, we accept and apologise that we are not there yet."