“We are talking to our European friends and partners in the expectation that by the early part of next year you will see an Article 50 letter. We will invoke that,” he told Britain’s Sky News television in New York.
Prime Minister Theresa May has previously said only that Britain would not trigger Article 50 before the end of this year.
Doing so would mark the formal start of a two-year negotiation period for Britain to leave the EU following its referendum vote in June to pull out of the 28-nation bloc.
But Johnson, who spearheaded the campaign for Brexit, indicated he did not think the negotiations would need the full two years to be completed.
“In that letter I am sure we will be setting out some parameters for how we propose to take this forward,” he added.
“I don’t think we will actually necessarily need to spend a full two years but let’s see how we go.”