* Western powers back India joining nuclear cartel
* China leads countries who oppose its inclusion
* Key group decides rules for civilian nuclear trade
By Fredrik Dahl
VIENNA, June 14 (Reuters) - Britain has stepped up efforts to let India join an influential global body controlling nuclear exports, a move that would boost New Delhi's standing as an atomic power but which has faced resistance from China and other countries.
The diplomatic tussle centres on whether emerging power India should be allowed into a key forum deciding rules for civilian nuclear trade, even though it has refused to join an international pact under which it would have to give up its nuclear weapons.
London, Washington, Paris and others argue nuclear-armed India should join the Nuclear Suppliers Group (NSG) - established in 1975 to ensure that civilian nuclear exports are not diverted for military purposes.
Britain has pressed its case in a paper prepared ahead of the NSG's annual meeting this week, arguing India qualifies because of the size of its civilian atomic industry and its commitment to stopping the spread of military material.
Western powers have taken a keen interest in the nuclear emergence of India - particularly its ambition to expand its capacity in the next 20 years by adding nearly 30 reactors, making it an attractive prospect for technology exporters.
But other NSG states have voiced doubt about accepting a member like India that built up a nuclear arsenal outside a global pact set up more than four decades ago to prevent countries from acquiring nuclear arms.
If India joined the NSG, it would be the only member of the suppliers group that has not signed up to the 1970 nuclear non-proliferation treaty (NPT).
Beijing's reservations are believed to be influenced by its ties to its ally Pakistan, India's rival, which has also tested atomic bombs and is also outside the NPT, analysts say.
India - Asia's third-largest economy - would need the support of all 48 NSG members to join the secretive cartel that regulates nuclear trade and has a key role in countering nuclear threats and proliferation.
But the body has remained split. "There is no unanimity on this issue," a senior official from one NSG state said.
The United States sealed a landmark civilian nuclear supply deal with India in 2008 that China and others found questionable because Delhi is outside the NPT.
It ended India's atomic isolation following its 1974 nuclear test and could mean billions of dollars in business for U.S. firms. Britain is also exploring a nuclear cooperation deal with India.
The British document, obtained by Reuters on Friday, stated: "The UK strongly supports India's accession to the NSG at the earliest appropriate moment."
"The UK believes that the NSG is best served by the inclusion and membership of India, with an important civil nuclear industry which continues to uphold the international non-proliferation architecture," the paper added.
It was not immediately clear how the British paper was received. A statement issued after the closed-door meeting in Prague said only the NSG's "relationship with India" was discussed. Officials had said they did not expect any decision already now.
At an informal meeting on the issue in Vienna in March, diplomats said China stressed the need for equal treatment in South Asia, an apparent reference to Pakistan.
Research assistant Daniel Painter of the International Institute for Strategic Studies (IISS) think-tank said that by joining the NSG India would have a voice in determining the group's new export guidelines.
But, he said, it would also threaten the NSG's credibility.
"It would further solidify the perception of India as an accepted non-NPT nuclear weapon state," he wrote in an analysis.
India and Pakistan - which have fought three wars - have both refused to sign the 189-nation NPT, which would oblige them to scrap nuclear weapons.
Pakistan - which has been trying to move closer to Asian powerhouse China as Islamabad's ties with Washington have suffered - has warned against allowing its rival into the NSG. (Editing by Andrew Heavens)