By Crispian Balmer

ROME (Reuters) - A former Italian premier who wrote the European Union divorce clause that Britain is poised to trigger said on Thursday that Brussels should offer no concessions to London in looming negotiations to quit the trading bloc.

"When it comes to the economy they have to lose," said Giuliano Amato, explaining that only then might the British reconsider abandoning the world's largest single market.

Britain voted in a referendum on June 23 to leave the EU. To do so, London will have to invoke Article 50 of the Lisbon Treaty, which starts a two-year countdown to a formal exit from the 28-nation bloc.

"I wrote Article 50, so I know it well," Amato told a conference in Rome, saying he had inserted it specifically to prevent the British from complaining that there was no clear cut, official way for them to bail out of the Union.

"My intention was that it should be a classic safety valve that was there, but never used. It is like having a fire extinguisher that should never have to be used. Instead, the fire happened."

Amato served as Italian prime minister in 1992-1993 and again in 2000-2001. He later helped draft a lofty European Constitution which eventually morphed into the less ambitious Lisbon Treaty.

A committed EU supporter, Amato said the so-called Brexit vote was a "disaster" and urged other European leaders not to follow the example of Britain's David Cameron, the prime minister who called last month's referendum. Cameron stepped down on July 13 and was replaced by Theresa May.

"If another leader is as mad as Cameron to offer a referendum on EU membership, for example in Holland or Austria, there is a risk (they would vote to quit)," Amato said.

To show Britain the error of its ways, Amato said the EU had to be "especially tough" in the Brexit talks.

"Don't give Britain the possibility of thinking that Brexit is a better way of doing what they have always done, grabbing what suits them (in the EU) and opting out of what they don't like. Brexit is a total opting out. They know this very well.

"The more they realize that they are losing, then the more chance there is that in 2020 someone will do something about it," he added, acknowledging that this was an "absurd hope".

Britain is due to hold national elections in 2020 and Amato said he hoped a party promoting a pro-European agenda might win power and put a brake on Brexit plans.

For that reason, he said it was important to stretch out the talks. "I hope that the negotiations are dragged on so they won't be wrapped up by 2020. (Prime Minister) May wants to wrap things up by 2019, but it will be easy to prolong matters."

(Reporting by Crispian Balmer; editing by Mark Heinrich)