Sky Sports’ fishing expert has claimed a trout allegedly caught in the Thames was actually taken from a freezer.

Keith Arthur, who presents the long-running fishing show Tight Lines, has cast doubt on the “monster” 10lb haul – arguing the fish in the photograph had been dead for some time.

The Wandsworth Guardian reported recently that the 10.2lb (4.6kg) trout was pulled out of the river near Putney Bridge by Kiwi brothers Dave and Ivan Shannon.

According to experts, the catch, although discoloured by pollution, was one of the largest seen in recent times.

But Mr Arthur, who also writes a column for fishing magazine the Angling Times, believed the unusual markings were actually burns caused by keeping the fish in a freezer.

He said: “Rainbow trout are not indigenous to UK rivers but are stocked in many lakes around the country, mostly on a put-and-take (kill what you catch) basis.

“While it is just possible such a big fish could have escaped, probably from Syon Park where there is a managed fishery, the only place that fish escaped from is a freezer.

“The discolouration so noticeable isn’t from pollution, it’s from ‘freezer-burn’ – that trout has been dead for a long time. Unless he took it from his freezer post mortem, and took it to the river for a picture, he is pulling a fast one.

“If it lived in the Thames it must be an excellent survivor because regular sewage discharges, 39m tonnes per annum into the tideway, have wiped out all aquatic life in the river from Isleworth to Putney and beyond at least twice in the past 12 months.”

Mr Arthur pointed out even if the fish was caught in the Thames, Mr Shannon broke the law by taking it from the river during the statutory close season – October 1 to March 31.

The Shannon brothers were contacted but were unavailable to respond to Mr Arthur.