May appointed Johnson as her "senior duty minister" while away on holidays, taking over from Chancellor Philip Hammond, who took on the role last week.
The appointment means that Johnson has been placed on standby to act as caretaker PM, should the situation warrant it, however Downing Street officials were keen to stress that May was still running the country.
Don’t panic but Boris Johnson is the most senior minister in Whitehall this week because everyone’s on holiday pic.twitter.com/Kd2NiON1uT— Emily Ashton (@elashton) August 15, 2016
"The Prime Minister is very much in charge and is constantly kept abreast of what is going on," a government spokesperson said.
'Larry the Cat Can Stop Him From Doing Anything Silly'
The news, while not expected to serve any functioning purpose, has been met with bemusement and humour by opposition political figures and the public alike, who have questioned Johnson's suitability for such a role.
"Putting Boris Johnson in charge of the country is like putting the Chuckle Brothers in charge of Newsnight," Liberal Democrats leader Tim Farron said.
Amidst all this Olympic Hoopla, you know Boris Johnson is in charge this week? No really. pic.twitter.com/qLrY42mZfw— Christian O'Connell (@OC) August 16, 2016
"Still, at least if he's here, he's not in Rio offending everyone he meets — and there's always Larry the cat to stop him doing anything silly."
People asking if Boris Johnson is running the country while Theresa May is away; happy to confirm he has to run all decisions past me.— Larry the Cat (@Number10cat) August 15, 2016
"Perhaps he can do something useful with his time in charge, like finding the £350 million a week for the NHS that he promised."
Apparently Boris Johnson is currently in charge of the country: / The good news is everyone can take their toys into work on Friday— Tim Burgess (@Tim_Burgess) August 15, 2016
Labour deputy leader Tom Watson also joined the criticism, saying: "Very few people could have imagined a short time ago that Boris Johnson would be in temporary charge of the country this week, including Mr Johnson himself.
"Perhaps that explains why he apparently has nothing to do, despite occupying one of the great offices of state."
Apparently with every other minister on holiday, Boris Johnson has been left in charge of the country. Like Homer Simpson running the plant.— Christopher Fowler (@Peculiar) August 15, 2016
"Many will suspect that, like his old school chum David Cameron, Mr Johnson craves high office but doesn't really know what to do with it when he achieves it."
Boris: A History of Insults
The former Mayor of London was a surprise choice for the role of foreign secretary in Theresa May's first cabinet as PM, particularly given his history of gaffes and offensive remarks aimed at those from other countries and cultures.
Earlier this year, Johnson won a poetry prize for his limerick about Turkish President Recep Tayyip Erdogan, in which he suggested the leader had had sex with a goat.
Boris Johnson in charge of country while May on holiday.. he is going to throwing mad parties at number 10 and trashing the place— Richard Fenton (@RichFenton) August 15, 2016
Johnson has also taken aim at allies much closer to the UK, accusing US President Barack Obama of harboring an "ancestral dislike" for Britain as a result of his "part-Kenyan" heritage.
The verbal attack came after Obama has said his preference was for the UK to remain in the EU.