Why is my Vauxhall Astra's fuel economy decreasing?

I have noticed a significant decrease in my Vauxhall Astra's fuel economy over the past few weeks. What could be causing this and what can I do to improve it?

It is possible that your fuel injectors may be clogged. I would recommend getting them professionally cleaned.

Have you checked your tire pressure lately? Under-inflated tires can greatly decrease fuel efficiency.

Another possibility could be a faulty oxygen sensor. I would suggest having it checked and replaced if necessary.

Have you been using a different brand of gas recently? It's possible that the gas you're using is not as high quality as your usual brand.