Rob Fusari has filed a 35 million US dollar (£23 million) lawsuit in New York City against the Grammy-winning singer, saying his protege and former girlfriend ditched him as her career soared.

Although he initially dismissed her, Fusari realised she had star potential after hearing her play in his New Jersey studio in 2006, the lawsuit said, and he spent the next few months working with her every day and "radically reshaping her approach", persuading her to drop rock riffs for dance beats.

As they co-wrote songs such as Paparazzi, he transformed her into Lady Gaga, the lawsuit said.

According to the lawsuit, Lady Gaga and Fusari's relationship turned romantic and became a business partnership in May 2006, when they created a joint venture called Team Love Child LLC to promote her career. Fusari's share was 20 per cent, it said.

Fusari says he introduced Lady Gaga to a record executive who ultimately shepherded her to Interscope Records, which released The Fame in 2008. The album has sold more than three million copies in the US; Fusari has a producing credit.

But the lawsuit says their personal and business relationship had soured by then and he has been denied a 20 per cent share of song royalties, 15 per cent of merchandising revenue and other money he is owed. He acknowledges getting cheques for about 611,000 US dollars (£400,855) but says that is not his full share.

Lady Gaga's spokesman, Dave Tomberlin, declined to comment on the suit.