Davis warned that “the clock is ticking” toward the March 2019 Brexit date and there is no point in negotiating aspects of the withdrawal deal twice.
“I firmly believe the early rounds of the negotiations have already demonstrated that many questions around our withdrawal are inextricably linked to our future relationship,” he wrote.
“Nowhere is that point truer than on the question of Northern Ireland. It is simply not possible to reach a near-final agreement on the border issue until we’ve begun to talk about how our broader future customs arrangement will work,” he added.
London is keen to start negotiating about its post-Brexit trade relationship with the EU, but Brussels has insisted that progress must be made on exit arrangements first.
Opening rounds of Brexit talks with the EU have made little progress, with European negotiators demanding greater clarity from the UK delegation.
The next round of talks is due at the end of August. Brexit talks formally began in June, a year after Britain's historic referendum vote to leave the 28-member European bloc.
Experts have warned it will be extremely difficult to negotiate a new UK-EU free trade agreement before Brexit, particularly as the EU has so far refused even to start trade discussions.
Britain’s membership in the EU customs union, which currently allows for the tariff-free movement of goods, will end along with its membership of the single market when it exits the bloc in March 2019.
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