ERROR: transaction is read-only

Hello all,
I am facing a serious issue in my app that uses jdbc and I am not
getting satisfactory answers from the posts either. I have set the conn
auto-commit to false but even after that I am getting the error : ERROR
: transaction is read-only - while executing a batch of 2 statements.

Statement1:
Smt.addBatch("DELETE from table1 where coln1 < 20");

Statement2:
Smt.addBatch("INSERT into table2 (SELECT coln1, coln2 from table3)");

Pls let me know how to resolve this.

Thanks in  advance,
-Satish