Dear Suresh,
The arrangement of outsourcing of labor being adopted in your company is nothing but an "Umbrella Contract." The salient feature of which is the continuity of employment of the same contract workmen in the same establishment with periodically changing the contractors for the purpose of records.
If you analyze the CLRA Act, 1970, you will find that engagement of contract labor on jobs or operations of perennial nature is prohibitable by the Appropriate Government. Even otherwise, in case of any complaint or claim regarding permanency or for absorption as regular workmen of your company by the contract labor, it would be very easy to prove that your contracts are sham or bogus ones only to deprive such workmen of the rights and benefits of regular employment under the garb of contract labor.
Such a practice is certainly an unfair labor practice under the Industrial Disputes Act, 1947 [Sl no 10 of I of Schedule V]. Any such judicial orders against your management would have a retrospective effect from the day one of the commencement of such an arrangement. So it would be prudent to take the issue seriously at least now and plan your staffing pattern accordingly. The ratio decidendi of the case laws and the periodical restrictions imposed by the Governments on contract labor nowadays indicate the trend that the adoption of contract labor is not more beneficial or better than regular employment but for the flexibility in Hire and Fire.