MUNICH (REUTERS) - A jailed German banker said on Friday he could not explain why Formula One boss Bernie Ecclestone made a multi-million dollar payment to him that is central to the prosecution case in the Briton's bribery trial.

Ecclestone is accused of bribing banker Gerhard Gribkowsky by channelling US$44 million (S$55 million) to him in return for smoothing the sale of a stake in Formula One held by bank BayernLB to the private equity firm CVC eight years ago.

The 83-year-old, who denies bribery charges, could face up to 10 years in prison if found guilty.

Gribkowsky, the former BayernLB chief risk officer, was jailed for 8-1/2 years in 2012 over the payments.

His relationship with Ecclestone is the central issue in a case that threatens to cost the Formula One boss his job and his reputation, and he was regarded as a key prosecution witness.

But Gribkowsky, appearing as a witness for the first time, struggled to give precise details on what had gone on.

Asked why he received money from Ecclestone, he told the court in Munich: "I never asked myself that question. I'm still annoyed with myself for that today."

Judge Peter Noll, who also presided over the case when Gribkowsky was convicted two years ago, was surprised that the former banker could not give a clearer explanation. "It's hard for me to comprehend (what went on) if you are unable to say more precisely how it came about," Noll told Gribkowsky.

Prosecutors allege that Ecclestone favoured CVC as the new owner because it was committed to keeping him on as chief executive of a business he had built into a global money spinner over the previous three decades.

Ecclestone admits making multi-million dollar payments to Gribkowsky but says this was to silence the German who he said was threatening to make false claims about his tax status that could have jeopardised his fortune.

Gribkowsky nodded to Ecclestone and smiled at him as he took the witness stand. Ecclestone, who was following proceedings through an interpreter, returned the smile.

Isn the opening exchanges, Gribkowsky explained how he and Ecclestone had clashed after BayernLB acquired a 47 per cent stake in Formula One following the collapse of the Kirch media group in 2002.

As the largest shareholder, the bank wanted to reduce Ecclestone's control of the business and tensions soon developed between the two men. "It was a case of building up pressure to resolve the situation as quickly as possible," said Gribkowsky of tactics BayernLB employed with Ecclestone. "Being unpleasant to reach your goal."

However, Gribkowsky said the two tried to ensure that they could maintain a civil working relationship. "We put our differences to one side in a professional way. I went with him to races," he said.

The case is scheduled to last until mid-September, with hearings only once or twice a week to allow Ecclestone to carry on running Formula One.