The British Foreign Secretary's personal mobile number was available on the internet for at least 11 years.
Dominic Raab's digits were discovered by a Googling Guardian reader, who eagerly informed the daily newspaper.
Raab, who deputises for Johnson when the PM is traveling or in hospital with COVID-19 has previously raised the spectre of cyberattacks from states outside the Western fold.
“This is a staggering lapse in security from a foreign secretary who, only last month, was lecturing NATO allies about the cybersecurity threat posed by authoritarian regimes," fumed Labour shadow foreign secretary Lisa Nandy, who called for an inquiry.
A security expert speculated that criminal elements — or Westminster lobbyists — could exploit the lapse.
"The wide availability of Mr Raab’s personal phone number must increase the risk that other states, or even criminal gangs, have been able to eavesdrop on his calls," said a former UK national security adviser, Peter Ricketts.
"It also means that anyone who happens to have had his phone number … is able to lobby the foreign secretary, bypassing the official channels which everyone else has to use," Ricketts ventured. "Anyone taking on a role as sensitive as this should in their own interests pay as much attention to online as to physical security."