NHS Choices advises:
If you need to feed your baby away from home, take with you:
- a measured amount of formula powder in a small, clean and dry container
- a vacuum flask of hot water that's just been boiled
- an empty sterilised feeding bottle with cap and retaining ring in place
The vacuum flask doesn't need to be sterilised, but should be clean, and only used for your baby. The boiling water should kill any bacteria present in the flask. If the flask is full and sealed, the water will stay above 70C for several hours.
Make up a fresh feed only when your baby needs it. The water must still be hot when you use it, to destroy any bacteria in the formula powder.
Remember to cool the bottle (with the lid on) under cold running water before you feed it to your baby.
Alternatively, you could use a carton of ready-to-feed liquid formula when you're away from home.