Notice is two rules stacked on each other, and almost every argument about it comes from reading only one. There is a statutory minimum in the Employment Rights Act 1996 that neither side can go below, and there is whatever your contract says, which can be longer but never shorter. Which one applies to you is simply whichever is greater, and the answer changes depending on who is giving the notice.
The statutory minimum, by length of service
Section 86 of the Employment Rights Act 1996 sets the notice an employer must give someone continuously employed for one month or more: not less than one week where continuous employment is under two years, not less than one week for each year where it is two years or more but under twelve, and not less than twelve weeks where it is twelve years or more. GOV.UK states the same scale for redundancy. Twelve weeks is the statutory ceiling, so a twenty year employee and a twelve year employee have the same statutory entitlement.
What the employee owes, which is much less
The scale above runs one way. Section 86(2) requires an employee continuously employed for one month or more to give not less than one week, and that is the whole statutory obligation however long they have been there. This asymmetry surprises people: a fifteen year employee is owed twelve weeks and owes one. What usually lengthens an employee's notice is the contract, not the Act, and that contractual period is enforceable in the ordinary way as a term of the agreement.
Is a notice period legally binding, and what happens if you go early
A notice period is a contractual term, so it binds in the same way the rest of the contract does, and section 86(3) makes clear that a contract cannot cut below the statutory floor. That does not mean anyone can force you to attend work. Leaving without working the notice is a breach of contract rather than an offence, and the practical consequences are usually loss of pay for the unworked part, a reference that records how you left, and in rare cases a claim for the employer's actual loss. Section 86(3) also allows either side to waive notice on any occasion, or to accept a payment in lieu of it.
How notice interacts with a redundancy
In a redundancy the notice is separate money from the redundancy payment, and GOV.UK says so directly: as well as statutory redundancy pay, the employer should either pay you through your notice period or pay you in lieu of notice. Notice pay is based on the average earned per week over the 12 weeks before the notice period starts, which is a different window from the one used for the redundancy calculation itself. Getting the two windows the wrong way round is a common source of a figure that does not reconcile.
Other questions people ask
Do I legally have to work my notice?
Usually yes, because it is a term of the contract, but section 86(3) lets either side waive notice on an occasion or accept a payment in lieu instead. In practice many employers prefer to pay in lieu than to have someone serve out a notice, and in a redundancy GOV.UK treats paying in lieu as one of the two normal options.
Can I leave my job without notice?
You can physically leave, and it is a contractual breach rather than an offence, but you would normally lose pay for the unworked part and the employer can in principle claim its actual loss. If you have been employed for a month or more, section 86(2) puts your statutory minimum at one week whatever your length of service.
My contract says four weeks but I have been here nine years. Which wins?
The statutory minimum does. Section 86(1)(b) gives you one week for each year of continuous employment between two and twelve years, so nine years means nine weeks, and section 86(3) means a shorter contractual term takes effect subject to that floor. A contract can only improve on the Act, never reduce it.