Ken Wilkins and Martin Bos arrived to open their Southbank Road newsagents to find the front window of the store had been smashed after a large rock had been thrown through it.
Ken believes that the perpetrators were trying to break into the store. He said: "What was thrown was a lump of concrete from a garden and so there were no fingerprints. The rock was six times as big as a house brick - I'm sure they were trying to break in."
Ken and Martin had installed shutters in March 2007, but then received a letter from Sefton Planning Department informing them that their security screens were in breach of local planning regulations.
They launched an appeal last October but were unsuccessful and had to remove the £2,000 security system.
Sefton Planning Department banned shutters outside shop windows four years ago in an attempt to make the area more attractive to tourists.