Wow, there are some people here with a hazy understanding of contract law.Regardless of what the law says, no company wants someone working for them who does not want to be there. Moreover, they do not want to spend the money to onboard you only to have to roll you off again.Write to them, telling them that you have changed your mind / circumstances / whatever. Nothing will happen, but just be aware that you are unlikely ever to get a job with that company in the future. Also remember that the consulting world is a small place -- you may come across these people again in your career, and they may remember that you wasted their time and left them in a lurch.