Billing in case of sub contract operations

Sir,

A Sub-contactor registered in Karnataka is engaged by a contractor registrered in Delhi for works contract in Karnataka and asked the sub-contractor to issue IGST invoices to Main Contractor in Delhi since he is not having any registered place in karnataka.

But some professional are saying that this purely a intra state supply and the sub-contractor has to charge SGST & CGST. 

Since the sub-contractor is no way connected to the main customer and getting payment from main contractor directly how can he charge SGST/CGST instead of IGST.

Please help...

Thanks & regards,

Gopalakrishna.