Theresa May was to give China’s President Xi a box set of the hit BBC nature series Blue Planet 2, with a personal message written by Sir David Attenborough.

The gift of the documentary, which proved highly popular in China, marked a trip during which London and Beijing tightened cooperation on environmental issues.

As well as joint-working to tackle plastic pollution and the illegal wildlife trade, Theresa May also secured assurances over the trips main purpose of increasing trade.

Some 80 million people were said to have enjoyed the sequel Blue Planet documentary in China narrated by renowned biologist Sir David.

But the programme, which had a heavy focus on the impact of plastic pollution in the oceans, also helped spark a more mainstream movement to tackle the problem.

UK officials said Ms May’s gift of the series, along with the hand-written note from its narrator, was a sign that Ms May saw the issue as a “shared agenda” for the two countries.

On Wednesday the Prime Minister boarded a specialised boat on the Yangtze River, which cleans plastic particles from the water polluted from inland China before it gets to the sea.

Blue Planet II – in pictures Show all 10 1 /10 Blue Planet II – in pictures Blue Planet II – in pictures Broadclub cuttlefish "A broadclub cuttlefish (Sepia latimanus) in Indonesia. Its skin contains millions of pigment cells with which it can create ever-changing colours and patterns to apparently mesmerise their prey." BBC Blue Planet II – in pictures Coral spawning on the Great Barrier Reef, Australia "Once a year, responding to cues from the cycle of the moon and the temperature of the water, corals simultaneously release their eggs and sperm into the ocean. Tiny coral larvae drift in the ocean currents, some for days, some for weeks, before sinking back down to the ocean floor and settling to become new corals." BBC Blue Planet II – in pictures An aggregation of marbled grouper (Epinephelus polyphekadion), French Polynesia "Thousands of groupers gather here in one of the few pristine spawning aggregations for this species remaining in the world. By taking up the position closest to a female, the male stands the best chance of fertilising her eggs." BBC Blue Planet II – in pictures Saddleback clownfish (Amphiprion polymnus) "A male saddleback clownfish uses all the strength it can muster to push a coconut shell to its anemone in Borneo. Unlike their reef relatives, saddleback clownfish live around anemones that are found in the sand, far away from the solid structure of the coral reef. Male saddleback clownfish must use their ingenuity to find a hard substrate upon which the females can lay their eggs." BBC Blue Planet II – in pictures Peters' monocle breams (Scolopsis affinis) "A group of Peters' monocle breams blow water-jets onto a hiding bobbit worm (Eunice aphroditois) to expose its ferocious jaws. In so doing, they alert other fish to the worm's location, ruining the predator’s chance of a surprise attack. This new behaviour was published for the first time by Jose Lachat and Daniel Haag-Wackernagel in 2016." BBC Blue Planet II – in pictures Green turtles (Chelonia mydas) Green turtles in Sipadan, Borneo, jostling for their place at a cleaning station. Here, turtles are serviced by blennies and surgeonfish who rid them of algal growth, parasites and dead skin. In return, these fish receive a nutritious meal. BBC Blue Planet II – in pictures Bumphead parrotfish (Bolbometopon muricatum) "Bumphead parrotfish feeding on coral and algae in Borneo. These parrotfish use their large forehead to ram corals, thereby breaking them into pieces that are more easily ingested. Each fish ingests over 5 tons of structural reef carbonates per year and after they digest the edible portions from the rock, they excrete it as sand, helping to create small islands and the sandy beaches." BBC Blue Planet II – in pictures Coral garden "A fish's-eye view of a coral garden on the Great Barrier Reef in Australia. New underwater probe lenses, developed for Blue Planet II, allow the viewer to immerse themselves into the coral reef city like never before." BBC Blue Planet II – in pictures A coral grouper (Plectropomus leopardus) "A coral grouper on the Great Barrier Reef in Northern Australia. Groupers use a gesture dubbed the ‘headstand signal’ to reach across the vertebrate-invertebrate divide and encourage another species to help it hunt. Gestures such as this are thought to only occur in the largest-brained species. The discovery of this behaviour in groupers indicates that some fish are able to think flexibly to achieve their goals." BBC Blue Planet II – in pictures Bigeye trevally (Caranx sexfasciatus) "A school of Bigeye trevally gather at the outer edge of the reef wall in Sipadan, Borneo. These trevally are a schooling species, widespread in the tropics and usually found in large- slow moving schools during the day. They eat a variety of smaller fish, crustaceans and marine invertebrates." BBC

Ms May has also announced a joint UK-China effort to strengthen UK/China cooperation tackling the ivory trade, with Border Force officers will share their expertise in identifying smuggled ivory.

Earlier in the day the Prime minister also gave a premium chess set from the British Museum to Premier Li Keqiang, the Chinese second in command, along with a bespoke Downing Street tea set for his wife.

Ms May then stood next to Premier Li at a joint news conference, which she told that she expected deals worth a some £9bn to be signed during the course of her three-day visit.

Agreements included an increase in trade in diary products and a lifting on the long-in-place ban on British beef – worth £500m alone over five years.

Blue Planet II: 'Coral reefs could be gone by the end of this century'

The Prime Minister also said they had agreed to carry out a joint trade and investment review to identify priorities for promoting growth in goods, services and investment – something which would also move the countries closer to a bilateral post-Brexit trade deal.

Mr Li said that their talks had delivered “substantive results” and that their bilateral relationship would not be affected by Britain's impending withdrawal from the EU.

"We will work to maintain the continued growth in our bilateral relations to take forward our relationship in the golden era," he said.

"The two-way opening up between China and the UK will go even further and China will open even wider to the UK.”

He explained talks had covered issues including human rights, the protection of intellectual property rights and overcapacity in the international steel market – with China repeating a promise to reduce production by a further 100 million tonnes by 2020.

The two leaders said that they had also agreed to co-operate more closely on the United Nations Security Council to uphold world peace and the international rules-based order - including in relation to North Korea.

After the conference the pair and their partners and officials shared a dinner of duck soup and black truffle followed by braised beef and broad beans, and then assorted pastries.