Speaking about the 1948 contract Nye Bevan, the founding father of the NHS, offered consultants, he said he had "stuffed their mouths with gold".
That gold was the ability to continue private practice - something senior doctors have carefully guarded since.
Down the years, several ministers have had a look at the issue, but concluded it was simply too tricky to tackle.
One former health secretary told me they thought consultants would simply walk away en masse if the government got tough.
There has been some movement. In fact, the current contract had a little nibble at restricting private practice, by insisting that any doctor doing fewer than 11 shifts (of four hours each) a week offers any extra work to the NHS first before accepting private work.
It has probably had some impact, but only a little. A full-time contract is 10 shifts, so it ties them to only an extra four hours a week before they are free to work in the private sector.
But after years of shying away from the issue, the government looks like it wants to tackle this head on. It is not being explicit in coming out and saying it wants curbs on private practice.
But the implications of the move by NHS England to publish earnings is clear: there is a suspicion that some doctors are raking it in and that affects their motivation when it comes to the NHS - both in terms of working extra hours for the health service and how productive they really want to be - and by publishing the details there is a hope it will make doctors think twice about how much they do.
Longer NHS waiting lists means more private work as either people will pay to go private or, and this is more common, the NHS has to pay for patients to be seen privately.
The private market is worth £4.6bn a year, according to market analysts Laing Buisson, with a quarter coming via NHS referrals.
At a time when money is tight and ministers want to get more seven-day services, tackling this is clearly an attractive proposition.
Although with the consultant contract currently in the process of being renegotiated (talks have slowed because of the junior doctor dispute), expect consultants to be hopping mad that their pay is once again in the headlines.
It promises to be the mother of all battles.
But if the register of private work is published, what will it show? What is known about the extent of private work is pretty sketchy.
A British Medical Association survey from 2009 suggested about half of doctors in the UK (it's not limited to England) did private work - but this was a fall from six in 10 when it was done several years before that.
Of those that did private work, a quarter earn under £10,000 a year, while one in six made in excess of £100,000.
This included private hospital work, alongside earnings from other sources such as providing advice to pharmaceutical companies and assessments for insurance companies.
But it must be said this was a self-selecting poll - doctors were not compelled to take part, so it is hard to gauge whether it was truly representative.
There are plenty of specialities where private work is not so easy to come by.
But there are others - orthopaedic surgery, urology, ophthalmology and anaesthetics to name just four - where there is plenty of work. It means for some, earnings can be extremely high.
Average basic salary for NHS work is £89,000 in England. But this ignores a number of ways doctors can make much more.
There are bonuses - called clinical excellence awards - which can be worth tens of thousands of pounds a year. Once awarded, they are difficult to take away.
And then there is overtime work. For some, this can be pretty lucrative - a BBC investigation earlier this year found one made £375,000 in a year.
Now that figure is very revealing. That consultant was thought to be on a special deal as they worked in an area where there is a shortage of consultants.
It meant the NHS was prepared to pay them close to what they could have got on the private market. All in all, it suggests there are consultants out there earning in excess of £500,000.
Read more from Nick
Follow Nick on Twitter
The fight over NHS consultants' private work - and that is exactly what this is - goes back to the very start of the NHS.