Dear Abhishek: LOI and Offer letter have the same value and it assumes a legally binding nature when it is accepted unconditionally. An LOI is only an offer, to make it binding on the offeror (the one making the offer) it needs to be accepted. The offer and acceptance has to be made by consenting adults with sound mind and the subject of the offer shall be legally valid. In other words it shall not be illegal. For example there cannot be a valid agreement to assault someone. Then there has to be a consideration, if X has agreed to do some work for Y, then there has to be an agreed upon consideration (price) for it. Without such consideration or mutual obligation there will be no binding contract.
So, once the offer letter is accepted it becomes binding contract and while acting on it, if the joining is delayed by the offeror, then there is breach of contract and one is entitled to damages & compensation thereof.