Shrewsbury Prison is the most overcrowded in England and Wales, Home Office figures show.
No more than 300 men should ever be held at the prison
The jail is at the very top limit of what is described as its maximum safe overcrowding level.
The prison, designed to house 182 inmates, was holding 300 men when the figures were compiled last month, the highest number of inmates allowed.
The jail has been criticised recently for its "dismal record" in preventing prisoner suicides.
'Like a warehouse'
A Prison Reform Trust spokesperson told BBC News that the jail was built to hold 182 men, but that only 92% of that capacity was currently available, meaning the ideal number of inmates there should be no more than 168.
She said it was possible that Shrewsbury Prison had become a "repository" for people who would ideally be receiving treatment for drugs problems elsewhere, or be given non-custodial sentences.
"Overcrowding also leads to very few opportunities for prisoners," she said.
"(A prison) can end up becoming a bit like a warehouse, just holding people until the end of their sentences.
Six apparent suicides
"When overcrowding goes up, there's an increase in self-inflicted deaths across the country."
The jail, known as The Dana, has previously been criticised by a Midlands MP for its "dismal" suicide record.
Stoke-on-Trent South MP George Stevenson questioned why so many inmates have died at The Dana after one of his constituents was found hanged there in September.
Mark Keeling's was the sixth apparent suicide at the jail inside a year.
Other Home Office figures released on Wednesday showed that there could be as many as 91,000 inmates in England and Wales by 2011. Current capacity is about 77,000.