service tax

Hi

I have my firm got registerted in the month of april2009- with service tax dept,. But  I have been in to buisness since 2005-06,06-07,07-08,08-09,- , for the last 4 financial year my turn over exceeeded 20 lacs,.

But I did late registration and started paying service tax from april 2009 onwards,

I received show cause notice on march 2010, asking me to pay the dues for the year 2005--06,07-08,08-09 period.

Now I decided to pay the dues with interests, for the last 4 financial year. 

whether I will asked to pay for  the penalty ? or only I have to pay with interests.

pl advise