By Andrew Hay

BARCELONA, Spain (Reuters) - World Trade Organisation (WTO) chief Pascal Lamy said on Thursday a breakthrough in troubled global free trade talks was still possible but he warned of growing risks of failure.

After nearly six years of negotiations to lift millions of people out of poverty and boost the global economy, chances of a deal before a summer deadline are fading, trade analysts warn.

Success or failure rests on the political will of big trading powers like Brazil, the United States and the European Union, said Lamy, formerly the EU's top negotiator in the Doha round of WTO talks.

"It is do-able, it's not yet done and we need political traction for a final breakthrough," Lamy told delegates at a Barcelona retail summit.

But he added: "Failure of the round is a scenario which we all need to have in mind. If we are really responsible, given the state of negotiations, we need to consider this scenario."

The White House loses fast-track trade negotiation powers in June that allow it to do deals without approval of Congress.

Many consider fast-track vital to the success of the Doha round and the the Democrat-controlled legislature may not renew the powers after they expire.

LITTLE HEADWAY

Simon Fraser, head of cabinet to EU Trade Commissioner Peter Mandelson, said if a deal was not reached by summer it would be difficult to get talks going again before the next U.S. presidential term.

Asked about the chances of failure of the whole round, he said: "It will go on if it's not completed."

Negotiations can make little headway until big powers break cover with whatever it is they can agree amongst themselves.

Core elements of any deal are the same as they were when Lamy called a halt to the talks last July, saying the leading WTO members needed time to review long-held positions.

The United States and the EU must accept deeper cuts to farm subsidies and tariffs, while leading developing states must open up markets more for manufactured and farm goods.

Lamy, seen by some as the poacher turned gamekeeper of international trade, said the United States needed to make a new offer on reducing trade-distorting agriculture subsidies for Doha to go to final negotiations at WTO headquarters in Geneva.

Other nations of the 150-state WTO are growing restless as the so-called G4 -- the EU, the United States, Brazil and India -- seek common ground in behind the scenes bilateral meetings.

"Those not invovled in the process of quiet diplomacy want to know what is being discussed and are saying they're willing to make their positions more flexible," Supachai Panitchpakdi, former WTO chief and now head of U.N. agency UNCTAD, said on the sidelines of the Barcelona meeting.

"I see a better than 50 percent chance of a deal by summer," added Panitchpakdi, known for his optimism on Doha.

With upcoming general elections in key powers France and the United States, leaders have little interest in doing politically sensitive deals, said Xavier Durieu, head of Eurocommerce, a group which monitors trade and retail issues in the EU.

"Optimism is difficult, these countries would be well advised to look at long-term benefits rather than the votes of a small percentage of citizens," said Durieu.