Originally Posted by Hansof
Is an employer allowed to mess up your chances of another job like this?
NO, They are not allowed to do that, however in this big bad capitalist driven world, it boils down to how much you are going to cost the current employer (in terms of replacement, risk, and so forth) by leaving and how much you are going to save the prospective employer by coming on board as soon as possible. In reality they can scupper your chances of moving by holding you to the agreement, it is after all a legal contract of employ.
If you are employed by a UK registered outfit, *and* you have a T&C that says 3 months on *both* sides of the agreement (i.e they can dismiss you with 3 months, and you must give 3 months), then you can only negotiate and hope to reduce your term by facilitation and request.
HOWEVER
If on the other hand (as has happened to me before), you have to give 3 months, and some smart ar5e HR nasty has put 2 weeks in as the companys liability, then it becomes what is legally known as "penalty" and you only have to give 2 weeks, ie the contract must be equitable and you can choose which one suits you.
Read your T&C's very carefully, and possibly seek legal advice should you wish to pursue it.