fuji said:
Your employer can change the terms of your employment any time they like, provided that they give you sufficient notice. If there is a negative material change in the terms of your employment, and perhaps a reduction in hours will count as that, you could claim "constructive dismissal".
Something to be aware of is that if you just "accept" the change in your hours and start taking Mondays off, it seriously hurts your ability to later claim constructive dismissal.
If you're going to claim constructive dismissal, you should immediately put the company on notice that you are not in agreement with the change in your terms of employment. If they refuse to renegotiate, you are better off quitting and then suing for constructive dismissal than you are to continue working.
Obviously the downside to this is that you have to quit your job, which is why a lot of people blindly accept the material changes in their terms and working conditions. It simply may not be worth suing over.
You may be better off trying to negotiate something better that satisfies the company's desire to have you take Monday off and your desire to work it. Perhaps every other Monday off. Sounds like the company has financial problems and is looking to make some cost savings. Just make sure they don't do it at your expense without your agreement.
Also re-read your employment contract. There may be some fine print in there that you're missing.
Finally, above all else,
speak to a lawyer specialized in employment law!!! You should be able to get a free assessment to at least let you know which direction you should be taking.
p.s., how much time is left on your contract?
p.p.s., it's VERY important you speak to a lawyer because employers take advantage of naive workers all the time. It's a case of you snooze you lose. If you don't know your rights, chances are excellent your perspective employer isn't going to educate you in them. That's up to you to find out. I've seen plenty of contracts go out to potential employees that were full of bullshit terms that I would never sign in a million years and that the employer would promptly remove if the worker pushed back on them. But the workers often don't push back so the employer gets a sweet deal. You need an AGENT looking out for YOUR interests if you're not knowledgeable enough to do so yourself (and most people aren't; employment law isn't the bailey-wick of most people). Some people use unions, some people use agents, some people use lawyers... just make sure you have someone who knows what they're doing.