Late Chinese leader Deng Xiaoping's vision of "one country, two systems" was the key that opened the door to the successful negotiations for the peaceful handover of Hong Kong to China, said a former aide to late British Prime Minister Margaret Thatcher.

"It was Deng Xiaoping's vision and concept ... and I think he gave great leadership to the achievement of that position, which was the foundation for the successful negotiations," Robin Butler, the former principal private secretary of Thatcher, told Xinhua upon Deng's 110th birthday.

In 1982, Butler met Deng for the first time when he accompanied Thatcher to Beijing to start the negotiations over the future of Hong Kong.

"At that time, there was quite a lot of tension over the issue between the UK and China, and so the first meeting which took place in the Great Hall of the People was quite tense," recalled Butler, who was awarded a life peerage in 1998 and now sits in the British House of Lords.

For two and a half hours in that three-hour meeting, it looked very difficult to reach an agreement, but with only half an hour to go, Deng and the British side agreed on a communique aimed at maintaining the stability and prosperity of Hong Kong, he said.

"That was the crucial point for the British, and that paved the way for ensuing talks to begin," the 76-year-old peer noted.

The negotiations went on between 1982 and 1984, and it was during the negotiations that Deng announced that there could be "one country, two systems," and that China could accept the capitalist system in Hong Kong.

What was novel about the "one country, two systems" was that it was the first time the Chinese government determined that it was possible for two different systems to coexist in one country, he said.

"That really opened the door to the successful negotiations for the handover," he stressed.

Butler met Deng for the second time when he returned with Thatcher in 1984 for the signing of the Sino-British Joint Declaration for the handover of Hong Kong in 1997.

"On that occasion, the atmosphere was much warmer, and Deng Xiaoping was very affable and friendly, and the British delegation was treated with honor," the seasoned civil servant told Xinhua.

Regarding Deng as a critical figure in the Britain-China relations, Butler said: "Until the issue of Hong Kong was solved, there couldn't have been good relations between China and the UK...and they finished with agreement and good relations between Britain and China."

Remembering Deng as a man with bright eyes and "a good sense of humor," the British peer said the Chinese leader "had lived through a very turbulent life and it seemed to have produced a man with a very broad and civilized outlook."

"I admired Deng Xiaoping because he had a very tough personal history. He'd been imprisoned. He came out from that, and he seemed to be a very humane man," Butler said.

"He had a background and experience which was beyond anything which I or most people in the West have shared...and that was what I regarded as very interesting about Deng Xiaoping," he said.