* Research suggest greater use of prison labour than thought
* Firms say the stocks, components sourced unwittingly
* IKEA apologised in 2012 for using forced labour
BERLIN, Jan 14 (Reuters) - Leading German firms including supermarket chain Aldi and carmaker Volkswagen stocked goods or used components made by forced labour in former Communist East Germany, a study has found, although both firms insist they did so unwittingly.
Research by the federal authority responsible for managing the archives of East Germany's Stasi secret police (BStU) uncovered a far broader use of prison labour during the 1970s and 1980s by Western firms than previously acknowledged.
The study could eventually help victims seek compensation.
In 2012 Swedish furniture giant IKEA apologised for using forced labour to make some of its furniture.
It, like thousands of other Western companies, once subcontracted production to state-controlled firms behind the Iron Curtain, where labour costs were lower. Some of those state-controlled firms turned to prisons to find a workforce.
"The study found that IKEA was just the tip of the iceberg," a spokeswoman for the BStU said. "Some prisoners were forced to work against their will and in circumstances that endangered their health."
An estimated 10 percent of East Germany's prisoners were locked up on political grounds. The state all but collapsed with the fall of the Berlin Wall in November 1989, and west and east Germany unified a year later.
German broadcaster ARD, which received an advance copy of the study, said discount retailer Aldi had sourced products from an East German stockingmaker that used women prisoners.
It also said carmaker Volkswagen had used parts such as fog lamps, tail lights and windscreen-wiper pumps in its Golf cars and transporter vans made by East German firm VEB Fahrzeugelektrik Ruhla, which East German documents showed had used prisoners.
Aldi said in a statement: "It is true that through trading with East Germany we had a business relationship with the state-owned stocking firm Esda Thalheim."
"It was only through research by Bavarian radio last year that we realised that Esda Thalheim had given some parts of its production to the Hoheneck women's prison," it said.
Aldi added that an employee at the time had visited Esda Thalheim in the mid-1980s and had seen nothing to indicate it outsourced any part of its production, let alone used a prison.
"We condemn the apparently common practice in East Germany of using prison populations for forced labour," the firm said.
Volkswagen said that it did not know then, or now, in which part of the East German firm which product was made, and it had no knowledge that prisoners may have been used.
"Volkswagen never initiated the use, or knowingly approved the use of prisoners in East German firms, let alone profited from this," it added.
Roland Jahn, head of the BStU, told ARD: "Whoever got involved with a dictatorship and did business with a dictatorship, could never be sure under what circumstances products were made."
He urged firms to open their archives and fund further research, which could eventually help to compensate those wronged. (Additional reporting by Emma Thomasson and Andreas Cremer; Editing by Alison Williams)