The 'Russian Roulette' singer is said to have confided in pals about her 'sexy little secret', revealing she has been getting closer to the 'Like I Love You' hitmaker since his relationship with Jessica Biel ran into trouble last year.

A source told the Daily Star newspaper: 'She's getting close to Justin again and it's getting hot.

'Rihanna called a friend and said she had a sexy little secret to share. She started laughing and said, 'Don't trip, but I have hooked up with Justin'.

'She said it was really good, hot and wild and that they have some of the best make out sessions she's ever experienced.'

However, despite her apparent liaisons with the 'Sexyback' singer, Rihanna – who worked with Justin on the track 'Rehab' for her album 'Good Girl Gone Bad' - is not said to be interested in a relationship.

The source added: 'She's only out with the physical with him. And it looks like she got it.'

Rihanna, 21, has previously played down rumours she has ever been romantically involved with Justin, 28, and last month she denied any romance between them.

She said: 'No, that rumour about Justin is incorrect. We worked together on my album. It's not like we have the closest friendship. We just worked together one time.'

Despite her denials, rumours have persisted, and the pair were said to have made a 'conscious effort' to avoid each other at recent parties.

Meanwhile, Rihanna has promised to reveal all her most intimate secrets in an autobiography, entitled 'The Last Girl On Earth' – set for release in June.