Leaving before end of notice period

[Horispondle]

Can anyone tell me what the consequences of leaving before the end of a notice period are?
I’m on 3 months notice and proposing to leave after 5 weeks notice. Line manager initially said he would do what he can to be flexible but now saying that I have to do it.
I made a case showing what work I had left to do any why I thought I could get it finished sooner so it would form a natural break. However they are now saying there are other things needing done in the department that weren’t my tasks before the notice was put in.
They aren’t replacing me and I have handover notes/manual written for anyone internally to pick up.
However, that’s all irrelevant I’m sure as I’d still be breaching my contract.
Is there likely to be much negative consequences if I went ahead and did it anyway?

are you leaving to go to a competitor? how senior is your role?
Unless the answer is yes / very, they're highly unlikely to seek specific performance of your contract as it would cost too much / take too long to be worth it, however if you will ever need a reference from them or are likely to come up against the same people in your industry, it's just not worth it.Coast it out.

Yes sorry should have mentioned. I have had an offer elsewhere and I (stupidly) thought I was on 1 months notice and had agreed a start date before realising I was on 3 months notice.
Not a competitor and different industry. I’m a junior manager with no headcount. Unlikely to come across these people again so long as the new job works out!
I’m not naturally confrontational and it’s making me anxious!

They could take you to court for breach of contract (this very very rarely happens). At the very least you will lose any good will / a decent reference from them as they can simply put in your reference refused to finish notice period and is unreleable (factually true since you refused to finish notice period).

People think companies cannot give bad references, they can they just need to prove that it is factually correct and saying OP is unrelable would be factual correct since she is NOT following through with her obligations and is no different then a random employee just simply not turning up for 1 month.