Mr Johnson emerged from No 10 shortly after Buckingham Palace broke news of the duke’s death.
His “back of a fag packet, staccato” speech “delivered in a shabby hair-do” was less less than British people “are entitled to expect,” reader Perry Gardner wrote in a letter to The Independent.
“Would it be too much to ask Mr Johnson to arrange to have his hairstyle modified to suit the position he holds in Her Majesty’s government?” asked another reader, Tom Lambe.
The comments were echoed on social media.
“Boris Johnson always looks such a mess,”said one Twitter user. “The state of his hair when giving his statement about Prince Philip was quite disrespectful.”
“Boris couldn’t even do his hair today of all days! The disrespect,” another viewer tweeted following Mr Johnson’s speech. “Prince Philip would not stand for this.”
Others speculated on what the duke would have said about the PM’s appearance.
“I hope Prince Philip was looking down from somewhere on Boris Johnson's earnest words and saying...'why can't he brush his bloody hair',” one wrote.
It is not the first time Mr Johnson has fallen foul of criticism for his appearance, after former Good Morning Britain host Piers Morgan branded his hair "messy" and "disrespectful" in June last year.
Mr Johnson apologised but insisted: “I do brush it. I have a brush in my office.”