Hi Jihad
It all depends on what the letter contains.
Either they can be completely meaningless or create a binding acceptance.
If the letter of intent/acceptance contains the following matters:
1. A clear description of the scope of work - say by reference to the bnid document.
2. The time to complete with start and completion dates
3. The money to be paid
4. A reference to the contract terms to be applied
Then you have a binding contract.
The contractor can accept the terms by starting work - no actual contract needs to be signed.
If any of the 4 sections are missing or unclear then there is no basis for a contract and the contractor should procede with caution.
Best regards
Mike Testro