New York State Attorney General Eliot Spitzer has subpoenaed the four major music labels in an in-depth probe to check they are not colluding to fix music download prices.

Warner Music, Sony BMG, EMI and Universal Music are all believed to have received subpoenas that demand they co-operate with the enquiry.

The news emerges as music labels prepare to negotiate new download deals with Apple, in which they are expected to attempt to secure the right to charge more for popular songs, a practice the labels - slammed by Apple CEO Steve Jobs as "greedy" - call "flexible pricing".

The Financial Times reported an industry insider, who said: "It's hard to see how there could be collusion in this area, although perhaps Mr. Spitzer is looking at the calls from a number of companies to change the pricing model going forward."