Can I decline a full time employment offer after signing?
If I receive an offer, sign it, and then get a counter offer from my current company; is it acceptable to decline the signed offer before the start date? Or does the new company have legal rights to take action against me?
In my opinion no this is not acceptable because you've made a decision to deliver. However, if the agreement was breached in the contract some how then yes it's ok then and only then to decline the offer
Depends on what the fine print said on the document you signed, usually the contract of employment only start on the day you actually start working and receive a formal letter of employment.
Your professional ethic plays a roll which might be significant for a decision.
Yes you can. Even if you start working for a company, you can resign during your probation period without a submitting official resignation letter.
It is not an ethical thing to do. However, if you are motivated by the offer that you received last. I would suggest you to do it if your new company agrees to terminate the contract.
In my experience, by resigning, you probably lost the good will you had in your previous company. Never ever accept a counter offer. Remind yourself why you wanted to leave.
If it was just for the money, you could have negotiated beforehand. If you didn't try, ask yourself why? If you tried and it didn't work, why do they offer more now when they could have before? Is it only to delay needing to hire somebody else and have a vacant position in the meantime? In that case, the counter-offered position will become untenable in just a few weeks or month as the company will have probably lost faith that you want to work with them.
I found this podcast really informative on that topic: https://www.manager-tools.com/2011/11/what-do-with-a-counter-offer. (I am in no way related to the podcast else that I am one of their regular listeners.)
Good luck with your decision.
The answer depends on what your contract states but my guess (based on experience) is that you would be in breach of contract and action could be taken should they so wish.
Your company believes in you and is obviously happy with your performance. One thing to consider, you gave notice so this may stay in the mind of the company. "Will he leave if another offer comes"? You have every right to turn down an offer with no ramifications other than upsetting a new company.