Johnson is expected to be on the US East Coast until at least Wednesday as he argues for supporting the Ukrainians to “win this year.”
It is not yet clear if Johnson informed Downing Street of his plans ahead of time.
On Tuesday, he was scheduled to address GOP lawmakers at the Capitol Hill Club, a private Republican social club in Washington.
The event was hosted at the club by Congressman Joe Wilson, a Republican on the House Foreign Affairs Committee.
A staunch supporter of Ukraine, Wilson has previously proposed erecting a bust of Ukrainian President Volodymyr Zelensky in the US Capitol.
But Mr. Johnson will face a tougher reception from some Republicans who now control the House of Representatives and oppose indefinite U.S. commitments to Ukraine.
Amid criticism of Johnson’s tour in Washington, some UK government sources have suggested that the former prime minister has a unique opportunity to press for continued US support.
Mr. Johnson remains highly popular among American conservatives and has closer ties than Mr. Sunak among his fellow Republicans.
