Doctors will have to reveal the amount of money they earn from private work under new plans to help bolster transparency.
NHS England chairman Sir Malcolm Grant told The Times that the extent of private work carried out had been "under the radar" for too long and "we are looking into something that is quite a touchy subject".
A spokesman for NHS England, which is trying to increase transparency, said every hospital is to publish a register of consultants' outside earnings from April in a drive to unearth potential conflicts of interest.
It is believed that about half of England's 46,000 NHS consultants do private work, on top of average earnings of £112,000 a year.
NHS England will be launching a major consultation on proposals to strengthen the management of NHS conflicts of interest and to clamp down on inappropriate behaviour.
It comes as a NHS review has raised fears that some senior doctors may be handing over too much work to junior colleagues or spending a lot of time on private work.
Sir Malcolm told The Times: "It's not an attempt to curb private work by consultants. Let's just have some transparency here. Much of what goes on in these communities is almost under the radar."