(a)Where a dealer who carries on the business of supplying goods in the course of execution of a works contract entered into by him (hereinafter referred to as a contractor) through another such dealer (hereinafter referred to as a sub-contractor) directly or otherwise, and the sub-contractor executes such works contract and each or either of them is liable to pay tax under this Act, then notwithstanding anything contained in this Act, the contractor and the sub-contractor shall be jointly and severally liable to pay tax in respect of transfer of property in goods whether as goods or in some other form involved in the execution of such works contract.