But unity is in short supply in the UK's fractious, anxious governing party and Johnson's vow of loyalty did not quell suspicions he covets the leadership.
Johnson told delegates to the Conservative annual conference that "the whole country owes (May) a debt for her steadfastness in taking Britain forward, as she will, to a great Brexit deal."
May laid out her plans for Britain's exit from the European Union in a speech last month in Florence "on whose every syllable I can tell you, the whole Cabinet is united," Johnson said.
With EU divorce negotiations proceeding at a snail's pace, Johnson has positioned himself as a champion of a clean-break "hard Brexit." He wants the UK to adopt a low-tax, low-regulation economy outside the EU's single market, says Britain must not pay to get tariff-free trade with the EU and insists that any post-Brexit transition period should not last "a second more" than two years.
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"It is time to be bold, and to seize the opportunities, and there is no country better placed than Britain," he said. Johnson's Brexit stance, tougher than May's stated position, has added to uncertainty for British businesses, who want to know whether they will keep easy access to the EU market and its population of nearly half a billion.
The British Chambers of Commerce warned that business people "are growing impatient with division and disorganization at the heart of the party of government." German politician Manfred Weber, head of the biggest party group in the European Parliament, implored May today to fire Johnson, "because we need a clear answer who is responsible for the British position."