The gaffe-prone UK politician praised a group of British businessmen willing to invest in Sirte, in north Libya. The coastal city was wrestled from the "Islamic State" militia last year.
"They got a brilliant vision […] to turn it into the next Dubai," he said at a Conservative party conference in Manchester.
"The only thing they have got to do is clear the dead bodies away," he added.
Reacting to Johnson's remarks, conservative MP Heidi Allen said Johnson should lose his job.
Labour's shadow foreign secretary Emily Thornberry said Johnson's comments were "shameful."
"For Boris Johnson to treat those deaths as a joke - a mere inconvenience before UK business people can turn the city into a beach resort - is unbelievably crass, callous and cruel," said Thornberry, also a deputy in the British parliament.
"There comes a time when the buffoonery needs to stop, because if Boris Johson thinks the bodies of those brave government soldiers and innocent civilians killed in Sirte are a suitable subject for throwaway humor, he does not belong in the office of foreign secretary," she added.
Thornberry's comments were echoed by Jo Swinson from the Liberal Democrats, who said that the comments were another proof that "Boris is not up to the job" as UK's top diplomat.
Prime Minister Theresa May "needs to get her house in order and sack him."
Boris Johnson's career has been marked by a series of blunders, with the politician sticking to his flamboyant style even after taking over as Foreign Secretary last year.
As an MP in 2002, he commented on the visit of the then-Prime Minister Tony Blair to Congo in a column for the Daily Telegraph.
"No doubt the AK47s will fall silent, and the pangas will stop their hacking of human flesh, and the tribal warriors will all break out in watermelon smiles to see the big white chief touch down in his big white British taxpayer-funded bird," he wrote.
Several months after his appointment as foreign secretary, Johnson told a newspaper Czech it was "bollocks" to claim that freedom of movement was one of the founding EU principles. Earlier this year, he said that EU leaders can "go whistle" over their demands for the UK to pay off the so-called "divorce bill" for leaving the EU.
The latest comments on Libya come only days after videos of Johnson in a Myanmar temple surfaced online. In it, the Foreign Secretary is reciting a colonial-era poem by Rudyard Kipling, including a verse about British soldiers returning to the former colony. The British ambassador is seen interjecting and telling Johnson the move was "not appropriate."
In the fallout of his remarks on Libya, however, Johnson struck back at his critics, saying it was a "shame" that people with no knowledge of Libya "want to play politics."
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"The reality there is that the clearing of corpses of Daesh fighters has been made much more difficult by IEDs and booby traps," he added, using an alternative acronym for the Islamic State.