Sir Leon, who will travel to China later this month for talks with government ministers in Beijing on trade links with Europe, told an influential audience of diplomats and industrialists at the Royal Institute of International Affairs in London that ifthe terms were right, the early membership of China and Taiwan would be in the interests of the world economy along with those of both countries.

It was the first time that Sir Leon, a former British cabinet minister who has been tipped in the past to become European Commission President, had spoken on links with China, and his powerful speech sent a message to GATT and its successor, the World Trade Organisation, that they should not be too rigid in denying entry.

''The EU would welcome conclusion of the membership process for China, and for Taiwan, this year if it is possible,'' he said.

''When East Asia is fully represented in the multilateral trade system, economic relations will be easier because we shall all be speaking the same language and applying the same rules.

''We will do all in our power to make it possible. But China must do more than half the work if we are to meet so ambitious a target. Nor can we subordinate substance to the timetable.'' Sir Leon said that his talks in Beijing would centre on how to conclude the accession process on time. The EU had clear views on the sort of protocol that would enable China to join.

''Successful Chinese membership means membership on a basis which gives full credit for the reforms accomplished to date,'' he said.

''But as long as further reforms have still to be achieved on the road to a Chinese market-based economic system, the terms of membership must provide a variety of transitional provisions.'' These include: A selective safeguard clause and the traditional maintenance of quotas.

A ceiling binding on import duties somewhere between 15 and 25 per cent.

Progressive exchange-rate liberalisation.

Progressive liberalisation of the right to engage in foreign trade and of the right to invest.

In an aside clearly aimed at the GATT authorities and others, Sir Leon said: ''These developments will, I am confident, arrive more quickly than many observers now expect.'' He insisted that co-operation with China and other East Asian countries had to go beyond economics, citing the familiar free society issues of cultural exchanges, good government and the maintenance of a civil society. He called for ''open minds'' in the East.

''We do not want to replace the ideological conflict of the Cold War with a fresh battle between East and West over, for example, democracy and human rights,'' he said.

''But there must be frank dialogue on these issues, and on that principle Europe insists in all its bilateral relationships - not as a pre-condition for other activities but as a necessary strand in the policy mix.'' Dialogue did not imply the unilateral imposition of subjective standards, Sir Leon said, but accusations of social or environmental dumping and counter-arguments or protectionism could only be avoided if there was dialogue.