German carmaker Opel on Tuesday pushed back the first step in its takeover by France's Peugeot, after workers raised hurdles over the future of a prized research centre.

Workers have blocked the firm's legal reorganisation, a vital first step in the merger process, local newspaper the Allgemeine Zeitung reported citing sources inside the firm.

A source close to the employee representatives told AFP that there were still questions to be negotiated between workers and managers at Opel and American parent company General Motors, while adding that the talks were not confrontational.

GM agreed in March to sell Opel to Peugeot and Citroen maker PSA, after suffering years of losses at the European venture.

But workers at the firm, a historic local champion at its main factory in Ruesselsheim, near Frankfurt in western Germany, insisted the new owners honour GM's wage and jobs guarantees covering the coming years before giving their green light.

PSA chief Carlos Tavares said on a visit to Germany in early April that all existing job and wage deals would be maintained.

To reach that goal, all collective bargaining agreements dealing with German Opel employees are to be transferred to a new legal entity, Opel Automobile GmbH.

Even once the transfer is made, PSA will still not have taken control of the firm.

This week's sticking point is the future of some  7,700 workers at Opel's prized Ruesselsheim research and development centre, which designs motors and new car models, the Allgemeine Zeitung reported.

The employee representatives will not move forward until PSA agrees to uphold a past guarantee from GM to keep commissioning projects from the research unit for the next three years.

"We are on a good path regarding the transition, although there are still some open questions to be cleared up," Opel communications chief Michael Goentgens told AFP, confirming only that "internal information events" planned for Monday and Tuesday had been delayed.

"We expect the merger with PSA to be completed in the second half of 2017," he added.

PSA's plan to buy Opel and British subsidiary Vauxhall from US-based GM for 1.3 billion euros ($1.45 billion) would create Europe's second-largest carmaker after Volkswagen.

Opel and Vauxhall employ some 35,600 workers between them at ten factories across Europe, around half of them at three plants in Germany.

The deal has yet to receive the green light from competition authorities.