Transfer of Lease

A piece of land was leased by the Govt. in favour of X for 90 years vide a registered lease deed. There is no express condition in the said lease deed prohibiting the lessee to transfer his leasehold rights to a transferee. Therefore, the lessee has assigned his interest in favour of Y without consent of the lessor by executing an agreement to sell and regd GPA. Whether such a transfer of rights is valid in the eye of law? Please advise.