Lincoln prison has been listed in the top 20 jails for overcrowding with 652 inmates in a jail designed for 436.
Figures released by the Prison Reform Trust showed the jail, on Greetwell Road, was rated as being 150 per cent overcrowded.
But John Webster, who is the chairman of the Independent Monitoring Board which oversees the jail, said it was "within operational capacity."
The trust said overcrowding at jails inhibited work to prevent re-offending.
The deputy director of the trust, Geoff Dobson, said it was designed to hold 436 but at the end of last month it was holding 652.
"What we've found is that 88 out of 140 prisons across England and Wales are officially overcrowded - which is not good news."
Mr Webster said: "We've never, ever had a complaint from a prisoner saying: 'I am in over-crowded conditions at Lincoln Prison'.
"So, to that extent, we are satisfied that it isn't the problem that the Prison Reform Trust would have you believe."