Can a candidate begin work as soon as he has signed the draft contract?

No. Signing the contract does not authorise the contractor to begin work on the contract. If he did so, he would breach the public contracts code. The contract does not come into effect until the contractor receives contract notification.

The notification is formally served on the addressee against a receipt, or sent by registered recorded delivery letter, or any other means allowing the date of delivery to be registered. In principle, the date of contract notification is that from which deadlines are measured. The execution of the contract begins at this date.