I'm leaving due to being treated poorly, I have been offered a better position somewhere else, it's a 4 week notice period but due to the start date at the new company I asked my work if they would accept 3 weeks notice, they agreed and confirmed my last day in writing, so then I confirmed my start date at the new company and signed the contract. I have now had someone higher up in management tell me that they need the full 4 weeks and they have told me I have to stay for the extra week. I can't. I've confirmed my start date in my new job and signed the contract. I'm leaving my old job and going straight into the new one.
Work are now telling me that I have to stay the extra week and that it's in my contract to give 4 weeks notice and if I don't they will take action against me.. but they agreed in writing my last day now they are telling me I have to stay a week longer. I've been a good employee for them for years and since I've handed in my notice I am being blamed for leaving them short staffed. I'm stressed.. can they take action against me if I leave on my planned date?