In an illustration of the conflicting forces at work, the Conservative government is facing pressure from dozens of its own lawmakers to make a decisive break with the EU, while a major business group insisted today that Britain must retain access to the bloc's markets and skilled workforce.
"We're going to have an unprecedented level of uncertainty and that's one of the factors causing many commentators to predict that there will be a slowing of economic growth," Chancellor of the Exchequer Philip Hammond said.
"We just have to plan to accommodate it," he said.
Prime Minister Theresa May's government has said it plans to trigger two years of formal exit talks with the EU by March 31, but insists it would be foolish to give away its negotiating position before then.
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The uncertainty has helped fuel economic jitters and driven down the value of the pound, amid claims that the government is deeply divided about how to approach Britain's EU exit.
The EU has long been a divisive issue for the Conservatives, and May is being squeezed between politicians demanding she apply the brakes to Brexit and those who want her to hurry up and make a decisive split.
Some 60 Conservative lawmakers urged May yesterday to take Britain out of the EU's customs union, which guarantees tariff-free trade within the bloc but imposes levies on goods from outside. They also want a guarantee Britain won't stay in the European Economic Area, which gives non-EU members such as Iceland and Norway access to the bloc's single market.
EU leaders insist Britain will have to leave the single market if it seeks to impose controls on immigration from EU nations, a red line for many supporters of Brexit.
Many businesses, however, say losing access to the single market would be devastating, especially for Britain's huge banking and financial services sector.