Parking giant NCP has apologised and launched an internal investigation after drivers claimed their cars were locked into the Tower Ramparts park after a performance at the Ipswich Regent over-ran.

The opening night of Sister Act at the Regent on Monday was delayed because of technical problems, and instead of finishing as planned at 10.30pm the final curtain call was at nearly 11pm.

Four families at the theatre had parked at the NCP Tower Ramparts car park behind Marks and Spencer which has a closing time of 11pm. One of the drivers said they arrived back at 11.03 to find an NCP member of staff closing the park.

The Ipswich mother told her story on Facebook: “The show over ran, finishing late. Got back to the car park at 11:03, the man locked up and left four couples in the carpark stranded.

“We begged him to just let us out, he said: ‘no you re all stuck here over night now.’ No compassion.”

She said two of the other couples in the car park had come from Clacton and Colchester.

Other users had complained to NCP about the closure – and one said that the company had apologised to him and told him the member of staff had been given a reprimand.

A spokeswoman for NCP said: “We were incredibly unhappy to hear the reports of the way our customers were dealt with when they arrived back at the car park and asked a member of our team to keep the car park open past normal closing hours, to allow them to exit.

“This is not acceptable and against all the training and guidelines we give our team, as a result we have launched an internal investigation into this matter and are taking it extremely seriously.

“We apologise profusely to any customer who wasn’t allowed exit and we would like to hear directly from them so we can reimburse them for their parking and any out of pocket expenses, as well as offer them our most sincere apologies.”

The Regent is run by the borough council and a spokeswoman said they would be talking to car park operators to try to let them know if a performance was likely to finish late.

She added: “The opening night’s performance started half an hour late because of technical issues and then overran by a corresponding period. We apologise for the inconvenience this caused. Other performances have not been affected.”