Why are plastic bags so bad?

Around 13bn plastic bags are given free to UK shoppers every year.
The bags can take between 400-1,000 years to break down, and like all forms of plastic they do not biodegrade. Instead they photo degrade, breaking down into smaller and smaller toxic bits that contaminate soil, waterways and oceans, entering the food chain when ingested by animals.
Many plastic bags end up as waste on our beaches, streets and parks. When a plastic bag enters the ocean it becomes a harmful piece of litter. Many marine animals mistake plastic bags for food and swallow them, with painful and often fatal consequences. Nearly 90% of floating marine litter is plastic.