Britons are due to vote on June 23 on whether to stay in the 28-nation bloc. If they decide to leave, EU rules state that Britain will have two years to hammer out the terms of its new relationship with the EU on issues such as trade and the status of foreign residents.
If by mid-2018, Britain has not agreed, its membership will simply end unless all the other EU states agree an extension.
"We have to negotiate our entry to the single market, we have to negotiate our future relationship with the EU and then we have to negotiate our trade treaties with all other countries. So there`s a lot to be done," Gus O`Donnell, cabinet secretary between 2005 and 2011, told BBC radio.
"Obviously, at the end of two years anything we haven`t negotiated has to be extended by unanimity of a vote excluding us, so that`s a bit scary."
A document from the British government in February suggested that a British exit could lead to 10 years or more of negotiations, causing uncertainty which would damage businesses trade and investment.
Campaigners who want Britain to leave the bloc say EU states would be keen to get a new deal quickly in order to maintain their exports to Britain. They also dispute whether Britain would be subject to the two-year rule for negotiating an exit.
O`Donnell said the complications of negotiating a British exit from the EU would be heightened by the timing of elections in 2017 in Germany and France where governments are facing their own challenges from anti-EU opponents.
"I`m afraid the politics works completely the wrong way for us," he said.
Ex-civil servant says UK would struggle to agree EU exit in 2 years
30 March 2016
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Britain would struggle to negotiate an exit from the European Union within a two-year deadline if voters decide to leave the bloc in June`s referendum, the country`s former top civil servant said on Wednesday.
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