German prosecutors on Monday said they had charged a lawyer and six employees of a now insolvent bank with fraud over a mass scam that saw them falsely claim nearly 400 million euros in tax rebates.

The accused allegedly conspired to take part in "cum-ex" transactions between 2006 and 2009 that saw investors reclaim tax that was never paid, Frankfurt prosecutors said in a statement.

The seven suspects - six Germans and one American aged 48 to 67 - face charges of "serious tax evasion" while an unnamed law firm is listed in the indictment as having played a supporting role in the trickery.

The Financial Times, citing "people familiar with events", earlier this month named the company as renowned law firm Freshfields Bruckhaus Deringer and said it faced a substantial fine if found guilty. It also said one of the accused was the former global head of tax at the firm, a German national who was arrested in November and released on bail the following month.

Freshfields declined to comment when contacted by AFP.

The prosecutors said the "cum-ex" trickery saw the suspects purchase "several billions of euros" in shares through Maple Bank in Frankfurt, which has since been declared insolvent.

The stock then quickly changed hands amongst the participants around dividend day, in order to claim multiple tax rebates on a single payout.

The accused allegedly deprived the state of 388.5 million euros ($430 million) worth of tax revenues, around 100 million of which has already been paid back.

Two British former investment bankers last month became the first suspects to face trial over a "cum-ex" scandal in Germany.