[quote=wilyum]@chemos,better offer n among other securities issues,logistics etc....[/quote]
And why did you accept this job in the first place?
If the reporting date is about or more than 4 weeks away, then you can send a regret notice. They have time to recruit another person, because, maybe, it is a crucial or critical post/project. If you are meant to report on Monday 4th June, and you decline now, you will paint yourself in very bad light.
Set out to correct the world's wrongs and you will most certainly wind up adding to them.