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If your employment agreement requires you to give three months notice, it is likely that you will be in breach of contract if you fail to work this notice period. Your employer cannot physically make you serve your three months notice but, if he decided to bring a claim for breach of contract, he could claim damages if he incurred financial loss as a result of the breach. However, your employer will also have to mitigate his loss, so he cannot just sit around for 3 months incurring losses and will be obliged to find a replacement (or temp staff), if necessary, to reduce these.
In reality, except when dealing with highly paid senior employees, it is quite rare for employers to bring a claim against an employee for breach of contract for failing to observe their notice requirements. This is because of the time and cost that the employer would have to devote to the claim. However, please be aware that even if your employer does not bring a claim for breach of contract, a failure to serve contractual notice may mean that he refuses to provide you with a reference.
Mindful of the risks above, if shortening your notice is unavoidable, you should contact your employer in writing at your earliest convenience, notifying them of your resignation (if not done already), setting out the reasons why you feel your employment is becoming untenable and seeking agreement regarding your early leaving date.
I hope this is helpful but please do not hesitate to let me know if you need any further clarification. I would also be very grateful if you could rate my answer as soon as you have chance, so that I can get credit from JustAnswer for my work.