There will be a massive political price to pay if the Cancun conference fails to make meaningful progress, Patricia Hewitt has warned.

Ahead of the World Trade Organisation's summit in Mexico, which starts on Wednesday, the trade secretary gave a frank assessment of the consequences if governments do not make a deal.

"If we are to fail, and I don't believe we will, it would be disastrous. We would have failed to offer real prospects to move out of poverty to move millions of people," Hewitt said.

"We would have also done probably irreversible damage to the WTO."

Hewitt will be leading the British delegation and will be joined by environment secretary Margaret Beckett, international development secretary Baroness Amos and trade minister Mike O'Brien.

Over five days ministers and lobbyists will be battling to reach a deal on key issues including agriculture subsidies, export controls and development.

Hewitt was upbeat on the chances of success ahead of the talks.

"The gains that could be made here are enormous," she said.

"A couple of months ago I was pessimistic. We simply weren't making progress. I am now much more optimistic."

Beyond the technical rules and tariffs, the trade secretary said a clear principle had to be established.

"What is clear is that the rich countries of the world cannot preach free trade aboard and protectionism at home," she said.

The summit will also try to reach agreement on the four issues of widening trade opportunities, government procurement, competition and investment.

The British team are highlighting coffee prices and the subsidies that lock out developing countries from trade markets such as the EU.

Off the agenda are the US tariffs imposed by the Bush administration on steel imports and US farming subsidies.

And ministers are in no doubt that the US response to any declaration will decide whether Cancun becomes a landmark or a laughing stock.

Officials confirm ministers have been talking with the Bush administration but remain tight-lipped on whether there has been any progress.

The summit is also expected to see the now-familiar protests by thousands of anti-globalisation protestors and the trade secretary

Hewitt argued that it could play the same role as the EU in "lifting millions out of poverty" and called on the campaigners to back the WTO.

"Don't knock and try to destroy an organisation which is trying to bring the world together and support countries you want to help.

Environment secretary Margaret Beckett said the recent decision by the EU to reform the common agricultural policy had set a strong precedent and described a WTO deal on agriculture as "long overdue".

"One of the things we have to do is show how far the EU has moved," she said.

"We now want to look at what we're doing on the global front and get some results. We have made in Europe a very big step on agriculture and we now need to see that matched not only by the US but by other countries."

Beckett argued that Britain's close relationship with both the US and the Commonwealth meant the UK would be a major player at Cancun.

"The UK is in a position to play a key role and have a useful input behind the scenes," she said.

Foreign Office minister Mike O'Brien admitted that reaching a final deal would mean tough dealing behind closed doors.

"We have to get agreement. We have to get everyone to sign up to this and that is going to be difficult. Everyone is going to be looking out for themselves. So we have a big problem ahead of us but it's going to be worthwhile," he said.