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.

Write to them, thank them very much for the offer and tell them how much you believe you'd have gained by working with them. Then tell them that you regret that you will not be able to work with them due to unforeseen circumstances that have cropped up and you hope that in future, you will be able to take up a position with them when another one comes up.

Usually, organisations are not just looking at you. there's the second best candidate. Do not jeopardize your position (chance of taking another position that is better for you) for the sake of another company.... Think about it, you think if the company was presented with a choice of you being a second best and another being the best they will give you a chance? Do what you feel is good for you career wise. As long as you are polite enough to let them know....

@Wilyum go by Wendz's advice.

While we are at it, would you ever accept a counter offer from your employer after resigning?

@ondiek,kul, under no circumstances, @nakujua,thxs.

I would say you go with Wendz advise. In any industry, the world is a very small place and you do not want to look and smell unprofessional. But you need tact in saying you got a "better offer".

I once had four job offers in quick succesion. I took one and after negotiations I accepted offer. Next day I'm called by another organisation I had applied to and had a better offer. I called the HR of the first org and explained and then followed up in writing. I had a month to report to the other job and after the morning introductions and what not, my team takes out the "new team member" into their lunch joint. As we are there, I get a call and its yet another offer for a position I had applied 13 months back. This was my hardest day in the world of employment. After consulting wide, I decided to take the new offer, whose package wasnt significantly different from the job I had just reported to but was rich in portfolio.

I had a candid talk with the HR lady, who was quite elderly and after the 30 minute talk, the agreement was I give notice of three weeks to allow them kujipanga. At the end of the day, always keep yourself at the tip of every consideration but maintain professionalism. Now I don't know for sure if wagging between 3 different jobs in the span of 25 days is professional but...you need to do what needs to be done.