MP Ian Liddell-Grainger said passengers were being left stranded when buses failed to arrive, while the ageing fleet of vehicles was frequently hit by breakdowns, causing further problems.
He said in one instance a woman travelling from Taunton to Minehead was forced to wait because her scheduled bus failed to turn up.
The replacement vehicle then broke down in Williton, and the next bus she caught only made it as far as Dragon’s Cross, Bilbrook, before it, too, broke down.
Mr Liddell-Grainger said: “That meant her regular 35-mile, two-hour journey from Curry Rivel turned into one lasting well over four hours.
“In another instance, a woman who arrived by train into Taunton also expecting to travel on to Minehead on the 28 was left stranded when the bus simply failed to materialise.
“The only way she could get home was by forking out a very large sum of money for a cab.”
Mr Liddell-Grainger, MP for Bridgwater and West Somerset, said he had frequently received complaints about the 28 service.
“A lot of them have related to passengers being unable to even get on buses in the centre of Minehead on a Friday or Monday because they have been full up with returning Butlin’s visitors,” he said.
“But this year the situation has become manifestly worse.
“First Group is treating passengers on this service with complete contempt.
‘It is operating with clapped-out vehicles which should have been sent to the scrapyard long since, and appears to be pleasing itself whether or not they run at all.
“For anybody in West Somerset who needs to get to a hospital appointment in Taunton or to make a scheduled onward journey by train or air this service is now so unreliable as to be utterly useless.
“And since there is no longer a bus station in Taunton it is now completely impossible for anyone to make a protest in person.
“It is the drivers who, quite unfairly, have to put up with passengers’ entirely justified anger.
“Meanwhile, the company hides behind an online service which apparently allows it to ignore every single complaint it receives while continuing to operate what must be the most nightmarish route in the country.”
A Buses of Somerset spokesman told the Free Press: “We would like to apologise to any customers who experience issues or delays with the number 28 bus between Taunton and Minehead.
“Although the vast majority of journeys have operated, regrettably we have had some services missing due to the national driver shortage.
“We continue to do all we can to recruit drivers and to fulfil our commitments to our customers.
“We would welcome applications to join our team and we offer flexible hours, good rates of pay, and full training is provided.”