Whatever your contract specifies for the probationary period. The most common figure is one week, sometimes two; senior probation contracts occasionally specify the same notice as post-probation. The statutory minimum from the employee's side is one week once you've been employed at least one month, and nothing before that. The contractual figure applies (and can't go below the statutory floor).
Probation notice is set entirely by your contract. UK law doesn't prescribe a probation-specific figure; it just sets the statutory floor that applies whether or not you're in probation. So the document to read is your offer letter or contract, specifically the section called 'Notice' or 'Termination'. Most contracts handle probation separately, with a shorter figure for the probationary period and a longer one afterwards.
The most common pattern is one week of notice during probation, rising to one month after probation completes. Senior or specialist probation contracts sometimes use two weeks during probation rising to three months afterwards. A minority of contracts use the same notice both during and after probation, which usually signals an employer who doesn't really differentiate the two periods or who wants protection equal to the post-probation level from day one.
If you're in your first month of employment, the statutory minimum is zero (statutory notice on either side kicks in at one month of service). Your contract might still specify a notice period during that first month, in which case the contractual figure applies. Many short-notice contracts during the first month read like 'one week or such shorter period as the parties may agree', which gives both sides flexibility for very-early-departure situations.
Notice from the employer to you during probation works to the same rule: the contractual figure or the statutory minimum, whichever is greater. The statutory minimum from the employer is also one week from one month of service. Some contracts give the employer a longer notice during probation than the employee has to give, occasionally as a way of compensating for the easier dismissal grounds employers have during this period.
The form of notice during probation is the same as any other notice: written, dated, and clearly stating your last working day. You don't need a long explanation, but a one-line reason is courteous (the role isn't a fit, you've accepted another offer, personal circumstances). Keep a copy for your records. Email is fine if your contract doesn't require a particular form.
What you give up by leaving during probation is mostly time toward longer-term protections. Statutory unfair dismissal protection requires two years of service. Statutory redundancy pay requires two years. Enhanced benefits (longer notice, contractual sick pay, pension matching) often start at the end of probation, so a probation departure means walking away from those. None of this affects your immediate legal right to leave; it just shapes the cost-benefit.
Some employers ask probationers leaving early to repay training costs, particularly for expensive courses or industry-funded qualifications. Repayment clauses are enforceable if they're proportionate (graduated reduction as time passes) and if they were in the contract you signed. Check whether anything in your contract creates a repayment obligation before resigning, particularly if you've completed training recently.
Practical recommendation: read the contract notice clause specifically before resigning during probation. Confirm the probation-period figure, the post-probation figure, and which one applies on the date you're planning to resign. Use the notice period calculator on this site to get the exact final working day. Submit written notice that references the correct contractual figure.
General information about UK employment law. For your specific situation, contact ACAS or an employment-law solicitor.