By Steve Schifferes BBC News Online economics reporter in Doha

The world's most populous country, China, was officially admitted to the World Trade Organisation on Saturday after a 15-year battle - a monumental change to the world trading system.

In a ceremony lasting just a few minutes, the 142 members of the World Trade Organisation unanimously approved the accession of the world's largest country into membership.

Loud applause and hugs between the Chinese delegation and the head of the WTO, Mike Moore, greeted the decision in the glittering conference hall in Doha, Qatar, where trade ministers are meeting to try and launch a new trade round.

An event of historic proportions for the world trading system

Kofi Annan, UN Secretary General

For its part, China pledged to work hard to ensure the success of the trade negotiations, and thanked the five heads of the WTO who have been in post since it began its struggle for membership in 1986.

But it put the WTO on notice that a trade round could only succeed if it addressed the gap between rich and poor nations, and ensured that all countries would gain from globalisation, as it staked its claim to lead the group of developing countries.

Chinese Trade Minister Shi Guangsheng that it supported a new round of trade negotiations "on the basis of full consideration of the interests and reasonable requests of developing countries."

The United States was the first to congratulate China on its membership.

US Trade Representative Robert Zoellick hailed the move as an historic step that would strengthen the WTO.

"I believe that as this century unfolds and people look back on this day, they will conclude that in admitting China to the WTO we took a decisive step in strengthening the global economic trading system," he said.

And he said everyone would benefit from the expanded access to markets and the expansion of a rules-based trading system.

Later, a US press briefing on China was delayed when a group of protesters drawn from the non-governmental organisations attending the conference blocked the entrance, shouting slogans like "the world is not for sale" and "Zoellick go home."

For his part, the EU trade commissioner Pascal Lamy said it was the end of a long and arduous road and an enormous achievement.

Competitive threat

China hopes that WTO membership will cement its commitment to economic reform, which has led to a rapid economic expansion in the past 20 years and an explosion of foreign investment.

But some other countries, particularly in Asia, fear that China may take away export markets as it expands its trade.

"Of course China is going to be very competitive, but having China competitive under rules, under a binding dispute mechanism, is, I would have thought, in the whole world's interests," Mike Moore, the head of the WTO, said.

Long Yongtu says opening China's markets benefits the whole world

Industrial countries, who have negotiated a wide range of deals opening Chinese markets in agriculture, telecoms and financial services, are hoping that China will live up to its commitments.

"Just like every member of the WTO, China will have to deliver on its commitments, and we will be watching this very carefully," Pascal Lamy, the EU trade commissioner, said.

China insists it will meet its obligations.

"As long as our market is open to the outside, the more economic growth we have and the better for the world," China's trade negotiator Long Yongtu told reporters.

China will become a formal member of the WTO 30 days after it approves the terms of membership and notifies the WTO secretariat.