Dear

Thank you very much for the proposal you submitted to us
on (date) in response to our request.

We are aware of the great deal of time and work that went
into your bid and therefore, regret to inform you that we
have awarded the contract to another firm. We based our
decision on (reason for rejecting bid)

We will certainly be inviting your firm to bid on our
future projects and thank you once again for your proposal.