A consequence of the lockdown periods is that employees may have driven fewer private miles in their company cars, particularly where they have not been driving to the office.
If they are to avoid being taxed on the provision of private fuel they need to fully reimburse their employer for the cost of private fuel by 6th July 2021 for the 2020/21 tax year.
Note that the CO2 emissions percentage for the car is multiplied by the £24,500 notional list price used to calculate the benefit. For example, a director driving a Mercedes Benz E200 saloon company car (CO2 emissions 169g per km) would be assessed on 37% = £9,065 for 2020/21. If they are a higher rate taxpayer then that means £3,626 tax. That would be an awful lot of private fuel!
In addition to the tax payable by the director on the provision of private fuel there would be £1,251 Class 1A national insurance contributions payable by the employer.
Note that the private fuel benefit is an all or nothing benefit. There must be full reimbursement by 6 July to eliminate the benefit. The simplest method would be to multiply private miles by the HMRC advisory fuel rate for the vehicle.