Although Downing St has not commented, the fact that the Mirror (and by default any other outlet that chooses to print/broadcast details) has been threatened with prosecution under Section 5 of the OSA suggests that it's more than just a lot of smoke.
I'm going to reserve judgement for the time being, but I will also note that it's interesting that this story is apparently leaked a week after the publication of Sir Christopher Meyer's book accuses Bliar of being W's poodle.