Bert Williams, 71, from Prenton, Wirral, caught the 93lb fish in the sea off Norway.
Mr Williams told the BBC he initially thought he had caught a dolphin and it took two anglers more than 25 minutes to land the fish off Soroya, Norway.
The catch is about 10lb off the world record for cod.
Shortly before, he had snagged his fishing line on the boat's engine and thought he'd done that again or "landed a dolphin" as a pod had just passed the boat.
Mr Williams, who began fishing when he was six years old, said: "To get the thing out of the water, I had to pace myself. I wouldn't have been able to lift it in on my own."
He said: "Two of the guides on the fishing trip helped lift it into the boat and it took 25 minutes."
Despite its size, Mr Williams said: "I have caught a marlin bigger - it was 180lb."
As well as being the biggest cod landed by a British angler, it is the biggest for a disabled angler - Mr Williams has a heart condition.
The world cod record, of about 104lbs, is held by a Swedish angler.
Paul Stevens, a guide who organised the trip with Sportquest holidays, said: "The previous day, I caught an 84lb cod," he said. "I held the British record for 24 hours!"
Mr Stevens described the atmosphere on the boat as "electric". "Everyone was buzzing it didn't matter who had caught it - it was just amazing to see such a big fish."
The record-breaking fish, which died after it was reeled in, will be moulded into a fibreglass replica and mounted at the firm's offices in Norfolk.
A fisherman is celebrating after landing the biggest cod ever caught by a British angler.