Reports of lumps of concrete falling on shoppers' cars prompted the third closure of the Castlepoint centre since it opened in 2003.
Retail union Usdaw said it had repeatedly warned the Castlepoint management about the problems with the car park for more than a year.
Usdaw area organiser Colin Davies said: 'We have been telling the management for over a year that there were serious problems with the car park, after our members reported lumps of concrete falling onto their cars. This is a very serious health and safety issue for our members and shoppers, so we welcome this closure while an investigation takes place.
'The closure of such a major centre in the pre-Christmas rush is clearly a drastic step but one we welcome. We are totally committed to working with the Health and Safety Executive to make sure this matter is resolved quickly so our members can get back to work in a safe environment.'
Marks & Spencer has one of its biggest stores located at the Castlepoint centre. Other tenants include Sainsbury's, which has already begun deploying staff to other stores. 'We would urge other retailers at Castlepoint to do the same.' said Davies.
Castlepoint's managers said they were working to resolve the situation, but could not say for how long the centre would be closed.
A statement said: 'The car park and centre will re-open once the immediate situation has been resolved and Castlepoint management are satisfied there are no safety concerns. We are acutely aware of how this will impact on shoppers and other visitors to the centre, especially at this time of year. We truly regret this situation. However, our prime concern is the health and safety of employees, shoppers and other visitors.'