Police have urged the public to use different pin codes at the gym after a prolific thief stole hundreds of pounds from a man he snooped on while entering a locker code that he also used for his bank card.

Last September Declan Murphy, 35, secretly watched the victim as he typed in a locker code at a luxury central London gym. After stealing the victim’s phone and bank card from the locker, Murphy then successfully guessed that the four-digit code on the card would be the same as the locker code.

He stole £500 from the account and bought himself time by damaging the victim’s phone by drenching it in water and preventing access to his banking app.

Murphy, from Bromley, was found to be using membership cards under false names to repeatedly access Third Space and Nuffield Health gyms, Isleworth crown court heard.

He also withdrew £1,750 using three different bank cards stolen from a different gym, and used a separate stolen card to try to withdraw £600.

The victims who lost money were eventually compensated by their banks, the court heard.

On Friday Murphy was jailed for two years and three months after pleading guilty to three counts of theft and 11 counts of fraud by false representation. The sentencing judge, Edward Connell described Murphy’s offending as “pre-planned” and “well-practiced”.

Police said the technique had been used by other criminals, and warned the public to always use different codes for lockers and bank cards.

DCI Kevin Ives, of City of London police, said: “People who visit gyms expect their belongings to be safe in their lockers and don’t tend to expect anyone to be looking over their shoulder and stealing their pin.

“Unfortunately, we know this is a method used by prolific criminals like Murphy.

“Our message is clear, do not have the same pin code for your locker as your phone or bank card pins.”

Murphy had targeted a Third Space luxury health club in the City of London, City of Londonpolice said. After the theft, the victim, who had joined the gym after his wife died from cancer, discovered that Murphy had phoned the bank pretending to be him, and was given internet access to the bank accounts.

The victim noticed a missed call from his bank, but struggled to return the call because of the water damage to his phone.

Murphy withdrew £500 from a cash machine, and later the same day, unsuccessfully tried to buy £5,400 worth of items from an Apple store. Murphy was £15,000 in debt and stole the cards to try to pay off what he owes, the sentencing hearing was told.

The victim told the court how the theft had increased his “sense of loneliness” after the death of his wife.

Murphy was also handed a criminal behaviour order banning him from visiting any Third Space or Nuffield Health gyms for six years.