I have accepted and signed an employment contract for a new job. However I've been offered a new job opportunity elsewhere which I would like to accept instead.

So my question is how binding is the contract that I have signed considering I have not yet started? Does consideration mean that the contract is only enforceable once I have started the job?

The contract is already formed. All you need to do is give as much notice under the contract to terminate it as is contractually required.

Practically speaking, of course, I would think it would be unlikely to be an issue if you haven't even started yet!

Yes just advise them that you have found something else and there shouldnt be a problem

Ok, though reading about "consideration" law I thought that maybe the contract would only start after I had actually started the job despite the signed contract as in theory I haven't received a benefit (payment) or suffered a detriment (loss of time)...

So until I have actually started work and suffered this detriment is the contract actually enforcable?

I'm probably wrong as my legal experience is currently a combined sum of about 5 minutes

I was taught similar to VillaBoy; that the contract is only executed when the employee turns up for work and the employer is able to provide the facilities for the the duties to be performed with.

Consideration is about whether or not there is a legal obligation to do something, not whether you have actually performed it. For example, an obligation to go to work in a week's time can be consideration.

However I'm sure they can get someone else, just let the employer know politely and as soon as you can.

In reality they are higly unikely to do anything about it. Be fast and be polite in telling them!

Consideration is about whether or not there is a legal obligation to do something, not whether you have actually performed it. For example, an obligation to go to work in a week's time can be consideration.

 

However I'm sure they can get someone else, just let the employer know politely and as soon as you can.

Indeed. There is consideration on both sides; you are agreeing to perform work, and in return receive pay for that work. Consideration can be both "executed" (already performed) or "executory" (a promise to do something in the future).