A security guard has been attacked by a man wielding a hammer as he filled an ATM machine with cash.
The guard was filling the machine at Westfield Post Office, Goole, when he was hit over the head.
Police said the thief escaped with a large sum of bank notes stored in a plastic box measuring approximately 18in by 9in.
He and two other men, all wearing black Lycra headgear, escaped in a small black car after the attack on Tuesday.
The thieves, described as white and wearing dark clothes, drove off towards Pasture Road in Goole.
The security guard was treated by paramedics at the scene but did not need to go to hospital.