Existing iPhone 3GS owners on O2, who want to upgrade to the iPhone 4 on June 24, are to be charged £20 per month remaining on their contract.

At least, according to an anonymous tip off Pocket-lint has just received by email: "...Wanted to give you a heads up. O2's 'deal' in the UK will be to offer existing 3GS customers an option to buy out their existing contracts for £20 per month that remains. Whether the handset will invoke a cost on top of that I don't know."

Now, while we can't validate that information at present, we have contacted industry insiders who believe that this is plausible: "Given O2's recent network issues, the move could be O2 desperately trying to cling on to the customers they currently have," a well-qualified source told us.

Should it be true, it's also great news for the first wave fanboys who, if they signed 18-month contracts last time around, will only need to pony up an extra £120 on top of any handset cost.

Considering their dedication, that means they can get the 32GB iPhone 4 for around £370, if the price of the device is £250 as expected. It seems a lot, but nowhere near the sums forked out last year.

UPDATE: This has now been confirmed by O2...