Britain was looking for stronger economic ties with China even before the agreement which would eventually hand it Hong Kong was signed in 1984, secret documents released Tuesday showed.

Ahead of late prime minister Margaret Thatcher's trip to Beijing in December that year to sign the deal, officials discussed how she should make the case for British business with China.

The files also revealed how China's then leader Deng Xiaoping reassured Thatcher that his country would keep its side of the agreement, based on the concept of "one country, two systems", adding that China "always honoured its commitments".

The documents were released by the National Archives in London under the 30-year rule, which allows previously secret government files to be made public after three decades.

Thatcher signed the Joint Declaration with China in December, agreeing to hand Hong Kong, which has this year seen weeks of pro-democracy protests against Beijing, over to China in 1997.

Around six weeks before her trip, the British embassy in Beijing wrote to the Foreign Office urging that Thatcher should "press for greater export and investment opportunities for British industry under the signboard of greater participation in China's modernisation."

It added: "It will be important however to handle this in public in such a way that Hong Kong opinion does not see our moves to develop trade as exploitation of the Hong Kong agreement."

In a nod to this concern, officials decided that Thatcher should not travel with a party of British businessmen to avoid suggestions "that we were now getting our prize for having sold out Hong Kong to the Chinese," Peter Ricketts, a senior Foreign Office civil servant, wrote to Thatcher advisor Charles Powell on November 16, 1984.

During the trip, business deals including around the development of a nuclear power station in Guangdong were discussed, the files reveal.

Thatcher was also assured by Deng, seen as the architect of China's economic reforms, over his country's intentions in Hong Kong.

"Some people harboured doubts about whether China would honour the agreement. Deng said he wished to inform the prime minister and the whole world that China always honoured its commitments," a British embassy note of the meeting said.