A man told by doctors he was obese plans to sue the hospital after finding out he actually had a malignant tumour growing in his stomach for nine years.
Mr Smithson underwent three operations at the Berkshire hospital
When the tumour was finally detected at the Royal Marsden Hospital in London and removed it weighed 55lb (25kg).
Trevor Smithson, 53, of Yateley, Hampshire, said he visited the Royal Berkshire Hospital, Reading, several times and even had three operations.
The hospital said it had received a complaint and was investigating.
Mr Smithson has now registered a complaint with the Healthcare Commission, the independent healthcare inspection body.
He said he intends to take legal action against the Royal Berkshire Hospital.
A spokeswoman from the hospital said: "Unfortunately we cannot comment specifically on Mr Smithson's case due to patient confidentiality.
"We take all complaints very seriously. We investigate them thoroughly and provide a detailed written response.
"We can confirm that we have responded to a formal complaint from Mr Smithson and have been notified by the Healthcare Commission that a request for an independent review has been received."
She said the Healthcare Commission had not decided yet whether to sanction an independent review.
Mr Smithson first went to the RBH in 1994, with circulation problems, but the growth was undetected.
In 2003, he was referred to the Royal Marsden and within a month the growth was removed.
He said: "Nobody ever bothered to do a scan and find out that there was a monstrous tumour in there.
"I'm very fortunate that it's a form of tumour which develops very slowly.
"I'm not blaming anybody for the fact I had that medical problem. The fact it was there for so long is absurd.
"They cut me from the V in my ribcage down to the pelvic bone and pulled it all back and took it out as one monstrous lump," he said.
A large liposarcoma, a slow-growing tumour which develops in fat tissue, was removed.
During the operation he also lost a kidney, a chunk of his colon and had to have a small intestine bypass.
Mr Smithson said the surgeon there had been "astonished" to find out that he had in fact been receiving medical care prior to his referral.
Mr Smithson, said the situation had affected his life to the extent he found it difficult to find jobs because "they probably thought I had a massive drink problem" due to his physical appearance.
"I remember going on the beach and in bars and people just looked and thought: 'Jesus Christ, he should do something about that'," he said.
But even after the operation the condition still hampers his life. He is only able to work part-time in his job as an aeronautical engineer due to peptic and spinal problems which he says are connected to the tumour.
Mr Smithson, who is still undergoing checks at the RMH, says the tumour will eventually grow back.