Edinburgh Zoo's controversial campaign to rent a pair of giant pandas from China next year has roped in Alex Salmond, the royal family and the Royal Bank of Scotland.

Internal government documents seen by the Sunday Herald reveal that the zoo has been exerting influence in high places in order to put pressure on the Chinese authorities to agree to export the endangered creatures.

But the plan - despite being viewed as "primarily a commercial transaction" - has become entangled in the UK's highly sensitive diplomatic relations with China in the run-up to the Olympics, which opened on Friday.

The revelations have prompted fierce attacks from animal welfare groups opposed to the plan.

"Giant pandas are being exploited as diplomatic pawns in a commercial deal," said Ross Minett, campaigns director for Advocates for Animals.

"Putting such wonderful wild animals behind bars in Edinburgh Zoo would be wrong, and this ludicrous and demeaning plan should be abandoned at once."

The Royal Zoological Society of Scotland, which runs Edinburgh Zoo, announced in May it was negotiating a loan of two pandas from the Chinese government. In return for exhibiting the animals for 10 years from 2009, the society would agree to pay fees towards panda conservation in China which could amount to millions of pounds.

Government correspondence released to Advocates for Animals under freedom of information law suggests, the group argues, that pandas could be used "in an unethical manner which reflects poorly on Scotland's reputation".

In February this year First Minister Alex Salmond wrote to the Chinese foreign minister, Yang Jiechi, giving the Scottish government's explicit backing to the zoo's plan. It would enhance the "warm relationship" between Scotland and China, he claimed.

In another letter, Salmond urged the foreign secretary, David Miliband, to lobby Chinese ministers. "This is primarily a commercial transaction which the Royal Bank of Scotland has agreed to underwrite," he wrote.

The zoo also has support from Buckingham Palace. A letter from the private secretary of the Princess Royal, Princess Anne, in April promised that she would pursue the panda import with the British ambassador in Beijing while attending the Olympic Games.

Prince Andrew, the Duke of York, had earlier raised the issue with the princess at the request of a senior official from the Royal Bank of Scotland, the letter said. It also disclosed that Princess Anne had written in support of the zoo's plan to the Chinese ambassador in London, Madame Fu Ying.

"The ambassador will take your case forward when she judges the time to be right but she explained that recent events concerning Tibet and the Olympic torch relay have complicated the issue," the princess's private secretary told the zoo.

China's rule of Tibet has been the focus of a series of protests by human rights demonstrators, including attempts over the last few months to disrupt the ceremonial torch relay in London, Paris and elsewhere.

Will Travers, of the Born Free Foundation, said: "The UK's approach seems stuck in the colonial era when animals were treated as no more than goods and chattels, there to serve often obscure objectives of so-called national interest."

Dr Richard Dixon, Scottish director of the WWF, the environmental organisation, said: "There is little direct conservation benefit to bringing pandas to Edinburgh Zoo. There is also no guarantee that the considerable fees the zoo will be paying will feed directly into panda conservation back in China."

The zoo's plan, however, was defended by the Scottish government, which thought that it would "enthuse" the people of Scotland. "Zoos have an important part to play in promoting the conservation of endangered species, and educating the public," said a government spokesman.

Edinburgh Zoo welcomed the widespread support it had received. "It plays a vital role in our ongoing negotiations to bring giant pandas to Scotland," said Iain Valentine, head of conservation for the Royal Zoological Society of Scotland.

"The society's primary concern is to participate in an international breeding and research programme to help secure the future of the species."

As many as 90% of giant pandas could have been affected by China's major earthquake in May, Valentine said.

He added: "This highlights the precarious situation of the wild population and reiterates the need for breeding and research programmes."