Mr Johnson said that “very substantial sums” could be freed up for spending on UK priorities.
Speaking ahead of talks with European Council president Donald Tusk at the G7 summit in Biarritz, France, Mr Johnson said he had detected a “change in mood” in the EU and was hopeful of starting fresh Brexit talks.
But he admitted that it was “touch and go” whether a deal could be reached by the 31 October deadline on which he has promised to take Britain out with or without an agreement.
The prime minister’s comments come amid reports that Downing Street lawyers have calculated that as little as £7 billion of the cash may be payable if the UK leaves the European Union without a withdrawal agreement.
The £39 billion settlement forms part of the agreement sealed by Theresa May last November and rejected three times by Parliament.
It covers outstanding UK contributions to EU projects approved during Britain’s membership as well as future pensions for Brussels staff.
But crucially it also includes membership dues for the two-year transition period envisaged in Ms May’s agreement, which would not take place in a no-deal scenario.
Speaking ahead of talks with European Council president Donald Tusk at th G7 summit in Biarritz, France, Mr Johnson said: “If we come out without an agreement it is certainly true that the £39 billion is no longer, strictly speaking, owed
“There will be very substantial sums available to our country to spend on our priorities
“It's not a threat it's a simple fact of, of reality.”
Asked whether he believed it would be possible to restart talks with the EU, which have been blocked by his insistence on Brussels giving up the controversial backstop for the Irish border, Mr Johnson said: “In the last few days there has been a change of mood in the EU.
“It is sensible now to get going and we will get going.”
He added: "It all depends on our EU friends and partners, I think in the last few days there has been a dawning realisation in Brussels and other European capital what the shape of the problem is for the UK.”