Chargeability of Service Tax

The service recipient and the service provider have mutually agreed to pay the service tax as per the rates in force at the time of raising of the invoice. However, the service provider does not have a ST registration no. The invoice was not raised until the ST Regn no. was received. In such circumstances, is the service recipient liable to pay the service tax on the payments due as per the contract even if the ST Regn no. of the service provider is not in force?