Is it safe to start work on a letter of intent?

Only if you have read it as carefully as a contract, because that is what it is. A letter of intent is a short document sent to let work start before the full contract is signed, usually a thin one drafted to protect the party who sent it rather than you. Before relying on it, check four things: exactly what work it authorises, the spending cap, meaning the maximum you will be paid, and what happens when you reach it; how either side can end it; and which terms govern the work you have already done if the full contract is never signed. The common way this goes wrong is momentum: the job quietly runs past the cap on a document nobody looked at again, and the fuller protection everyone assumed was coming never arrived. So stop and reassess as you near the cap, rather than working on in the hope the main contract lands.

Say a piling contractor is asked to start on a letter of intent capped at £50,000 while the full contract is finalised. Work begins, the main contract keeps slipping, and nobody looks at the letter again. By the time the job is going well they have carried out £120,000 of work, and the full contract is never signed. Now they are arguing over the £70,000 spent above the cap on a document that expressly limited payment to 50,000. Had they stopped at the cap and refused to press on until either the letter was raised or the main contract signed, the exposure would have been a fraction of that. Momentum, not the wording, did the damage.