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It's a big day for the UK's only giant pandas.

The famous pair have left Scotland's Edinburgh Zoo to head back to China, after 12 years.

Pandas Yang Guang and Tian Tian first came to the zoo back in 2011 as part of a 10-year agreement between the Royal Zoological Society of Scotland (RZSS) and the China Wildlife Conservation Association.

It had been hoped female panda Tian Tian and male Yang Guang would breed while they were at the zoo, but they weren't able to produce a cub.

Their time in the UK was extended by another two years, but the time has now come for the pandas to return to China.

So, what's actually involved with getting the pandas ready for their long journey home?

"There's a whole lot of logistics that have to happen,'' says Darren McGarry, who is the head of living collections at the Royal Zoological Society of Scotland (RZSS), which runs the zoo.

The zoo has spent a long time making complicated plans to safely send the pandas to China.

They've had to be in quarantine since the start of November in line with animal health regulations and will also spend time in quarantine when they arrive in China, where they will live at a sanctuary in Sichuan province's capital, Chengdu.

Before their trip, the pandas will have regular vet checks, including blood and poo sampling, to ensure they are healthy and do not take any diseases into China.

They'll each travel in a specially designed metal crate on the flight which they've been taking some time to get used to, and will be accompanied by a keeper from Edinburgh Zoo and an RZSS vet.

Each crate is 190cm long, 146cm high and 127cm wide.

"Although they look small, there's actually quite a bit of room for them inside, it's not tight" said the zoo's blacksmith Rab Clark. "I think they'll be fine. I'm sure they'll have a safe journey."

A lorry will take the pandas from the zoo to Edinburgh Airport and their flight back to China will be on a specially hired plane which has had most of the seats taken out.

There are only four passenger seats for the small team helping with the big move behind the pilot and co-pilot.

The pandas will have health checks, as well as food and water during the 12 to13 hour flight.

Half way through the journey, the keys for the panda crates will be handed over to the Chinese keeper and they'll officially become the responsibility of China once again.

RZSS keeper Michael Livingstone, who'll be accompanying the pandas on the flight, says the pandas are usually a bit lazy and like a lie-in in the morning. He's been gradually bringing forward their wake-up time to get them used to earlier starts.

''It's bizarre, I feel strange." Mr McGarry said. "I've always known they were leaving and we try not to get attached, but we have to care for them so we are emotionally attached.

"It's a difficult day. I'm excited because I will go back for a follow up next year to make sure they're ok. I'm sure they will be. They're China's national treasure."