A total of 400 landmarks across the UK, including Edinburgh Castle, the Clyde arc bridge, the Forth Rail Bridge and the Kelpies in Falkirk, took part in the global event.
The WWF wants governments across the UK to implement policies to tackle the problem, including a “latte levy” on single-use drinks cups, and a deposit return scheme to encourage plastic bottle recycling.
To mark Earth Hour, WWF is also asking the public to make a “promise for the planet” - a small step in their own lives to help reduce their environmental footprint - such as refusing plastic cutlery or carrying a reusable coffee cup.
The predictions for rising plastic use in the UK are based on modelling existing available data and take into account factors such as rising economic output and the impact of new European legislation.
This year alone, the UK will use 10.8 billion wet wipes, 16.5 billion pieces of plastic cutlery, 42 billion straws and 4.1 billion single-use drinks cups and lids, the report said.
Tanya Steele, chief executive of WWF said: “We are polluting our world with plastic, suffocating our oceans and overwhelming our wildlife.
“The amount of plastic which the UK is throwing away is set to rocket by over a million tonnes by 2030 - that’s the equivalent of 87,000 more double decker buses worth of plastic waste each year.
“We must act now - banning avoidable single use plastic by 2025 - and introducing incentives to help people and businesses make the right choices to reduce, reuse and recycle.
“This Earth Hour, millions of people around the UK are sending a powerful message that we must act now on climate change and pollution to save our planet.”
The report also warned that while plastic waste would rise dramatically without action, recycling rates for plastic would increase more slowly.
Less than a third (31%) of plastic waste is recycled, a figure which is expected to rise to 42% by 2030, while single-use plastic recycling rates are predicted to increase from 29% to 37% over the next 12 years.
Earth Hour takes place at 8.30pm UK time on Saturday, with landmarks including Tower Bridge, Brighton Pier, and Cardiff Castle, along with millions of people, turning off the lights to mark the moment.
Across the world, landmarks ranging from Sydney Opera House to the Eiffel Tower in Paris are joining the global switch-off.
A Defra spokesman said: “We want to be the first generation to leave the environment in a better state than we found it, and we have already taken significant steps to tackle plastic through banning microbeads and taking nine billion plastic bags out of circulation with our 5p charge.
“But we all need to do more, which is why we have committed to eliminate all avoidable plastic waste and recently launched a call for evidence on how changes to the tax system can reduce waste from single-use plastics.”
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