Latest NHS Digital data show that patients are waited more than the national average before receiving treatment in emergency departments run by United Lincolnshire Hospitals Trust.
Unison officials said the figures were “shocking” and urged the government to take action to fund the health service.
Health bosses at ULHT said the trust’s performance is “not where we want it to be”.
The data, which reveals waiting times for October 2019, show that the average wait for someone in A&E in the region is one hour and 24 minutes against a national average of one hour and five minutes.
Meanwhile, five per cent of patients in Lincolnshire left A&E before being seen for treatment – more than double the national average of two per cent.
Elliot Dean, East Midlands regional organiser for Unison, said the country faced a “health care crisis” and urged the government to take action.
“These shocking new figures show the NHS is on its knees across Lincolnshire,” he said.
“Dedicated NHS staff are going above and beyond to care for the public. But they are under enormous pressure, with heavier workloads and too few staff.
“The government must take urgent action to fully fund health and social care services across the county before irreparable damage is done to vital public services and the people who rely upon them.”
Simon Evans, director of operations at ULHT, apologised for the trust’s A&E performance and said it was not at a level that officials wanted it to be.
“Over recent months we have seen a significant increase in the number of very poorly patients attending at our A&Es, as well as cases of the winter vomiting bug norovirus and more recently flu,” he said.
“This has coincided with a reduction in the number of patients being discharged from our hospitals, which has unfortunately resulted in long delays for some of our patients in A&E.
“We are disappointed by our recent performance, as during September we saw an improvement in our A&E waiting times and we were recognised as one of the most improved in the country.
“We know both regionally and nationally that October and November presented significant challenges for the NHS and this was reflected in our performance in October and November.
“We are working with partner health and social care organisations to ensure our patients receive the appropriate care as soon as possible.”
The trust has failed to hit its key waiting time targets for A&E for more than half a decade.
The last time ULHT achieved 95 per cent of patients being treated within four hours of arrival at A&E was September 2014, the same month it surpassed the 85 per cent target to start cancer treatment within 62 days of urgent GP referral.
A total of 15,139 patients came through A&E doors across the county in October alone, compared with 13,996 at the same time last year.
During campaigning for this year’s General Election, the health service was one of the key issues voters wanted to see addressed.
Labour shadow chancellor John McDonnell said on a visit to Lincoln that the local health service was “underfunded” and that staff were “under pressure all the time”.
Prime Minister Boris Johnson is expected to unveil plans to increase NHS spending in the upcoming Queen’s Speech.
The Conservatives have pledged an extra £33.9 billion a year for the health service by 2023/24.