LONDON — Britain should be ready to leave the European Union if Prime Minister David Cameron fails to achieve sufficient reforms before a referendum promised by the end of 2017, London Mayor Boris Johnson will say this week.

Johnson will on Aug. 6 introduce a report by his economics adviser Gerard Lyons which will say that staying in an unreformed EU would be worse for Britain than pulling out and retaining an open trade policy, the mayor’s office said.

“The U.K. can only achieve serious reform if it is serious about leaving,” Lyons said in an interview with the Sunday Times newspaper. “It can only be serious about leaving if it believes this is a better outcome than the status quo of staying in an unreformed EU. This report concludes that that is the case.”

Johnson, who is among the favorites to succeed Cameron as leader of the Conservative Party, will back the premier’s plan to renegotiate Britain’s membership and pursue reform before calling a referendum, the newspaper said. Johnson may announce his intention to stand for a seat in Parliament’s lower House of Commons in the next five weeks, the Times reported, citing unidentified allies.

Lyons’ report says that a million jobs could be created in London in the next 20 years under a reformed EU, while 900,000 would be created if the U.K. was to pull out and keep an outward-looking trade policy, Johnson’s office confirmed. Maintaining the status quo would create 200,000 jobs in the capital over the same period and withdrawal into isolation would cost 1.2 million jobs.

“These numbers demonstrate that, while securing significant EU reforms produces the best outcome for London and the broader U.K., leaving the EU and pursuing our own trade reforms is far better than the status quo,” Lyons told the Sunday Telegraph. “The EU’s policies toward the City of London form an important part of the reforms we’d like to see.”