The European Union would insist on completing a swift divorce with Britain before starting to forge any new relationship if UK voters decide to leave the 28-member bloc.
Two EU sources familiar with the bloc's latest thinking on a possible Brexit said there was no appetite to grant any extension of the two years provided by the EU's Lisbon Treaty for negotiating a withdrawal, while any new trade partnership would take many more years to conclude.
The stark view from Brussels means Britain could initially be cut adrift without any preferential relationship with its biggest trade partner.
This contrasts with suggestions by "Leave"campaigners that London could secure a special status preserving market access before it formally leaves the EU.
Top EU officials say they are still confident that Britons will ultimately vote in a 23 June referendum to stay in the community they joined in 1973, despite opinion polls showing a close race.
However in case of a "Leave" vote, the European Commission has tentative plans to hold a rare Sunday meeting on 26 June to set its strategy, one source said.
EU leaders would hold a brief summit with Britain two days later, at which Brtain would be expected to give formal notice to quit.
The 27 other states would then meet without British representatives to decide how to conduct the withdrawal negotiations and take the union forward, based on proposals from the executive Commission.
"It is in our interest to do the divorce as quickly aspossible. There's no appetite for negotiating new terms in the first two years," one source said.
Another source said: "The shorter the better. No one wants to go beyond the two years. The show must go on."
Both sources spoke on condition of anonymity because of the acute political sensitivity of any contingency planning for a British departure.
The first source said the initial two-year negotiations would cover only issues such as residual EU budget payments to and from Britain, the pensions of British EU civil servants and relocation of EU agencies based in the UK.
The EU treaty says exit talks can be extended by unanimous vote among all the member states, but both sources said London's partners would want a speedy separation.
Britain would probably leave on 1 July 2018, and become a"third country" in EU parlance. Formal negotiations on some form of trade and partnership agreement could begin only then, the first source said.
In case of a vote to remain, the Commission has kept in place a small UK Task Force which helped to craft a deal negotiated by Prime Minister David Cameron in February to meet British concerns on migrant workers' welfare rights and relations with the euro single currency zone.
The seven-member team led by British EU official Jonathan Faull would oversee smooth implementation of the agreement.
That deal lapses automatically if Britain votes to leave, and the first source said a completely different team would be appointed, probably led by a German or French official, to handle exit negotiations.