On this page
The official UK answer
GOV.UK says that when a vehicle is compatible with E10, there is no reason it cannot mix E10 and E5 in the same tank. It is also safe to fill with E5 when E10 is unavailable.
E5 and E10 labels mean up to 5% and up to 10% ethanol, not exact percentages in every litre. The blend depends on the actual fuels and amounts added; the labels alone do not give its exact ethanol content.
Check compatibility before making E10 the routine grade
Most petrol vehicles can use E10, and all cars built since 2011 are compatible according to government guidance. Some older, classic and specific early-2000s vehicles may require E5.
Use the official manufacturer checker, your owner's manual or the manufacturer. Diesel and electric vehicles cannot use E10 petrol.
What if E10 goes into a non-compatible petrol vehicle?
Official guidance says a single tank should not be a major problem: fill with the correct E5 grade next time. Unlike putting petrol in a diesel vehicle, a one-off E10 fill in a non-compatible petrol vehicle should not normally require draining.
Prolonged use may cause harm and is not recommended, so follow the official advice and vehicle documentation.
Do not confuse E-grade with octane
E5 and E10 describe ethanol content; check which RON your car needs separately before choosing a fuel.
E5 and E10 describe ethanol content. In the UK, standard E10 is generally 95 RON and super E5 is generally RON 97+. Do not use a simple blend calculation to justify buying below the minimum octane your vehicle requires. Follow the fuel and octane guidance in the owner's manual.