Childcare vouchers are currently being phased out by the government and replaced by a new government offer of “tax-free childcare”. They are only available to employees who had already joined the scheme and had their wages adjusted on or before 4 October 2018. You can’t use both childcare vouchers and tax-free childcare.
Childcare vouchers are offered to existing employees either on top or instead of a portion of their salary (known as ‘salary sacrifice’). Vouchers can affect your eligibility for Universal Credit.
You can find more information on the website of the charity Working Families.