A vital safeguard to protect the world's rainforests from being cut down has been dropped from a global deforestation treaty due to be signed at the climate summit in Copenhagen in December.

Under proposals due to be ratified at the summit, countries which cut down rainforests and convert them to plantations of trees such as oil palms would still be able to classify the result as forest and could receive millions of dollars meant for preserving them.

An earlier version of the text ruled out such a conversion but has been deleted, and the EU delegation - headed by Britain - has blocked its reinsertion.

Environmentalists say plantations are in no way a substitute for the lost natural forest in terms of wildlife, water production or, crucially, as a store of the carbon dioxide which is emitted into the atmosphere when forests are destroyed and intensifies climate change.

Now they are calling on Britain to take a lead in restoring the anti-plantations safeguard at the final negotiating session next week, saying that otherwise the agreement - which seeks to halve global deforestation rates by 2020 - will be fatally flawed.

"It is a priority for the safeguard to be reinserted, otherwise we will have a situation where countries are paid for converting their natural forests into palm plantations," said Emily Brickell, climate and forests officer for the Worldwide Fund for Nature (WWF-UK).

"If this is not changed, the agreement will be part of the problem, not part of the solution, because it will allow things to carry on as they are now and we will continue to see the loss of natural rainforest," added Simon Counsell, of the Rainforest Foundation.

The key piece of text which was lost said that parties to the treaty "shall protect biological diversity, including safeguards against the conversion of natural forests to forest plantations".

It was deleted in closed negotiations but some observers think it was done at the instigation of African rainforest countries, such as the Democratic Republic of the Congo and Cameroon, while other states including Indonesia and Malaysia are believed to have supported it.

Both are heavily involved in the oil palm industry, which is a major driver of deforestation because palm oil is used to make biofuels.

A move to reinsert the clause was blocked at the last talks in Bangkok by British officials, who feared that the gains of the week's negotiations (the text was reduced from 19 pages to nine) would be lost if the text were reopened.

Green campaigners accept that this was a matter of procedure but think it will have been a disastrously bad call if officials do not move swiftly to replace the lost text at the final negotiations in Barcelona, beginning next week.

"The EU has to make sure the wording goes back in," said Charlie Kronik, of Greenpeace. "It's absolutely essential, otherwise it leaves open the possibility of removing intact, high-value forests and replacing them with oil palms as party of the treaty."