A SITE manager has spoken of his shock at discovering that a piece of metal he was keeping on top of a filing cabinet was an unexploded Second World War hand grenade.

Martin Lee had been working for Wimpey Homes on a housing development in Alfreton Road, Derby, when the device was discovered during digging.

It was found on Friday and spent the weekend on top of a filing cabinet in his office until one of his workers said that it looked like a bomb he had seen on a television programme.

Yesterday, Mr Lee called the police, who sent for a bomb squad from Chilwell Barracks.

They destroyed the grenade in a controlled explosion at Derby Racecourse at about 12.15pm.

Mr Lee said: "We didn't know what it was. I thought it might have been a ball-cock because it was about the same size.

"It was on the filing cabinet over the weekend but we moved it out to the car park when we became suspicious. It's a mistake anyone could have made."

A Ministry of Defence spokeswoman said the 300g Mills hand grenade could easily be mistaken for a ball-cock.

She said: "Second World War ordinance is found regularly around the country.

"It's difficult to speculate on its blast range because it would depend where it landed – hard or soft ground, that kind of thing."

A police spokesman said Alfreton Road was closed for two hours while the grenade was moved to the Racecourse so it could be detonated in an open space.

He said people living and working in the area were told they could leave their homes if they wanted but that, once they had done so, they would not be allowed back in until the grenade was removed.

He said: "A cordon was set up for a 200m radius – which is a medium-risk.

"No-one was evacuated from their homes but about 60 people chose to leave."

Among those who could not return to work because of road blocks was Wendy Wilson, a delivery driver at PCS on Alfreton Road.

She said: "We were out making a delivery but were told we couldn't go back.

"Everyone inside the building worked as usual. They were told they could leave if they wanted to but the building wasn't evacuated."

PC Steve Reid, one of 10 officers manning the scene, said drivers had been told to find another route.