Hundreds of patients were waiting for routine treatment at Cornwall Partnership Trust in April, figures show.
A think tank has warned the Government is "moving in the opposite direction" from its goal of bringing waiting lists down.
NHS England figures show 309 patients were waiting for non-urgent elective operations or treatment at Cornwall Partnership NHS Foundation Trust at the end of April – up from 298 in March, but a decrease on 769 in April 2022.
Of those, one had been waiting for longer than a year.
The median waiting time from referral at an NHS Trust to treatment at Cornwall Partnership Trust was four weeks at the end of April – the same as in March.
Nationally, 7.4 million people were waiting to start treatment at the end of April.
Dr Sarah Scobie, acting director of research at the Nuffield Trust think tank said: "Yet another month of hitting a record high for the NHS waiting list will be very worrying for the government and shows it is moving in the opposite direction from its ambition to bring down waiting lists by 2025.
She said the charity is concerned by the speed and scale at which the backlog is growing.
"The NHS and its staff are throwing more and more resources at recovering the backlog as the summer months approach, but challenges, including industrial action and staff absences, make keeping up with the weight of demand incredibly difficult," she added.
Separate figures show 1.6 million patients in England were waiting for a key diagnostic test in April – the same as in March.
At Cornwall Partnership Trust, 118 patients were waiting for urodynamics tests.
Of them, 97 (82%) had been waiting for at least six weeks.
Cancer Research UK’s director of evidence and implementation Naser Turabi said: "Today’s data shows that all cancer waiting time targets in England have been missed and are amongst the worst on record.
"Despite the tireless work of NHS staff, people affected by cancer continue to experience unacceptable delays for vital diagnosis and treatment.
"We need to see urgent political leadership and action on cancer, and urge the UK Government to publish the long-awaited workforce plan for England," he added.
NHS England said staff are continuing to make progress on recovering routine services despite the it facing "the most disruptive industrial action in its history in April", with four days of strikes by junior doctors resulting in more than 195,000 appointments and procedures having to be rescheduled.
Accident and emergency units across the country also experienced the busiest May on record, it said.