Hobbyists have been urged to stop fishing with super-strength magnets after guns and grenades were plucked from the bottom of Britain's canals.

Magnet fishing has attracted an increasing number of enthusiasts after the unusual hobby was highlighted by England rugby star James Haskell earlier this year.

But authorities are now cautioning against the activity after a number of dangerous objects were dredged up from Britain’s rivers and canals including several unexploded World War Two grenades and an Uzi submachine gun.

In June, Martin Andrews, 43, and his son Jack, 19, died while magnet fishing in a canal in Huddersfield. An inquest was opened and adjourned at Bradford Coroner's Court.

The pair had only recently discovered magnet fishing - and had bonded over collecting many old an unusual items, but the alarm was raised when a dog-walker found their belongings on a nearby bank.

The Canal and River Trust said the practice is outlawed under a 53-year-old byelaw but that its approach was to refrain from taking legal action when dealing with first-time offenders.