Luton and Dunstable Hospital left the swab in Frank Hibbard's pelvis while removing his prostate in 2001.
The inquest in Ampthill, Bedfordshire, heard a mass the size of a large grapefruit, which had grown around the calcified swab, was found in 2014.
Mr Hibbard, 69, of Luton, died two weeks later.
Coroner Tom Osborne heard a foreign body was found on a scan in 2003 and recorded on Mr Hibbard's medical notes, but no action was taken.
Over the intervening years, Mr Hibbard complained of severe pain and was regularly prescribed painkillers by his GP.
The mass was eventually discovered during an operation, supposedly for a hernia, in 2014. He died two weeks later on 13 July.
The coroner concluded the death was caused by the combination of radiotherapy after prostate cancer and the cancer which had grown around the mass.
His widow Christine, who is suing the hospital, told the BBC ahead of the inquest: "It would have been our 50th wedding anniversary last year, it would have been his 70th birthday.
"We should have been enjoying retirement.
"I am waiting for answers, because I haven't had any."
In a statement released after the inquest, the hospital said: "We apologise sincerely to Mr Hibbard's family for the error that took place in 2001, and for the fact that a key opportunity to identify and remove the swab was missed during a CT scan in 2003.
"This is clearly something that should never have happened."
It said it had introduced a safe surgery checklist to minimise the possibility of it happening again.
A medical swab left inside a man during surgery contributed to his death from cancer 13 years later, a coroner has said.