Five months after a factory building collapsed in Bangladesh with the loss of more than 1,100 lives, a BBC documentary claims to have evidence of ongoing worker and safety concerns at facilities making clothes for western retailers.

An investigation for the Panorama programme "Dying For A Bargain" - which is being aired tonight (23 September) - claims factory workers in Bangladesh are forced to work 19-hour days in dangerous conditions.

Panorama secretly filmed workers making clothes for the supermarket Lidl, who were locked inside a factory in the middle of the night. Workers earned just GBP2 (US$3.2) for a shift lasting 19 hours.

They started work at 7am and were eventually let out at 2.30am. But when the BBC's reporter visited the factory posing as a western buyer, he was given timesheets that falsely claimed the shift ended nine hours earlier.

The programme is also said to have found another factory, making clothes for retailers like Gap and H&M, which was also hiding long working hours.

Most retailers have codes of conduct that limit the number of working hours and they are supposed to be enforced with regular audits.

But Kalpona Akter, from the Bangladesh Center for Worker Solidarity, says many factories hide the truth about long hours from western retailers.

"The factory owners, they keep two different books. So one they show to the buyers, the other they show to the worker. These retailers' so-called audits really don't work."

Panorama investigated factory safety following the Rana Plaza building collapse in which more than 1,100 people died.

The programme claims there have been more than 50 factory fires in the past ten months. Many workers have died because factory gates were locked and they were unable to escape.

Lidl said the programme's findings were "concerning" and showed how important it was to improve conditions in Bangladesh.

"Change, however, takes time and constitutes not only a challenge for Lidl but for all active companies in the retail industry."

The retailer said it had invested more than GBP6m (US$9.6m) to improve the living and working conditions in Bangladesh.

The factory, however, denied that workers were forced to work 19-hour shifts and said that a second gate in the building was open. It said the programme's allegations about time sheets were "false and baseless" and that the factory works legally and does not deprive workers of their rights.

Panorama also monitored working hours at a factory making clothes for H&M and Gap, where the working day sometimes started at 7am and finished at 10.30pm, but not all of the overtime hours were shown on the workers' payslips.

When the programme's undercover reporter visited the factory as a buyer, he was told that the working day was just ten hours.

The factory later admitted it had two sets of books, but said it was compliant with the law and that workers were never forced to do overtime. It also said some employees were working additional hours in order to extend their holidays.

H&M said overtime "remains a major challenge in the garment industry," which it monitors by checking documentation and interviewing workers.

Gap said its clothes made up less than 5% of the factory's output and that it has no plans to place further orders. It also said that suppliers are required to comply with its code of conduct.

The programme also questions another British retailer, Edinburgh Woollen Mill, which has not paid compensation following a factory fire in which more than 100 people died.

Boxes of Edinburgh Woollen Mill clothes were photographed at Tazreen Fashion following the fire last November, but the company said its clothes were made by another factory in the same group and had been stored at Tazreen without its knowledge or prior approval.

However, documents given to Panorama appear to show that Edinburgh Woollen Mill clothes were manufactured at the factory. They include specific product codes and give details about Edinburgh Woollen Mill T-shirts and polo shirts being made and inspected inside the factory.

The product codes on the documents match the product codes for clothing currently on sale in the company's shops.

The programme also spoke to former Tazreen workers who said they had been working on Edinburgh Woollen Mill products for months.

But the company strongly objected to the workers' claims. It said the paperwork was "inaccurate or fabricated" and that documents and clothes that had been stored at the factory were scattered around after the fire to imply that Edinburgh Woollen Mill products were made there.

The is not the first time Panorama has turned its attention on the garment industry. A documentary aired in 2008 claimed to show children making garments for fashion retailer Primark - but was exposed two years later for using fake footage.