The planned upgrade of the tracks, signalling and rolling stock, which will cost billions of pounds, is being examined by the regulator.
Richard Branson, owner of Virgin Group which owns 41 per cent of Virgin Rail, said: "The sign-off of the upgrade has to be done before any announcement. We have to sign-off the deals and they have to be in place first. A few ticks are needed and then we are ready to go."
One company source later added that the rail regulator could make a final decision on the rail upgrade "within a few weeks".
That will free Virgin to make its float decision, but it refused to elaborate. The market expected a float in June, but this timetable is now certain to slip to later in the summer.
If Virgin goes ahead with the float, industry sources indicated it could be worth pounds 250m.
Other shareholders in Virgin rail include JP Morgan and Bankers Trust. Virgin refused to comment on whether it was considering a retail offering, but it said it was confident it could secure backing if the float went ahead.