Jonny May scored an early try for England at Twickenham on Saturday but Fiji -- who had lost all seven of their previous Tests against the Red Rose brigade -- largely outplayed their hosts for most of the match as Borthwick's men suffered a fifth defeat in six games.
It was the first time England had lost to a current tier-two nation.
Fiji, many of whose players now appear in leading club competitions in the southern hemisphere, England and France, went ahead through second-half tries from Waisea Nayacalevu and Vinaya Habosi.
England hit back with tries by Marcus Smith and Joe Marchant, but any thoughts of a fightback were dashed when the composed Fiji scored another try in the 73rd minute, Simione Kuruvoli sealing a famous victory as the Pacific islanders sent a warning to their World Cup pool opponents.
England, the 2019 losing finalists, head into their tough tournament opener against Argentina in Marseille on September 9 at a low ebb.
"I am sure every England supporter is disappointed with that today," Borthwick told Amazon Prime.
"It is important to congratulate Fiji. I thought they played exceptionally well and took their opportunities.
But, he added, England had been the authors of their own misfortunes.
"When they (Fiji) scored they scored very easily. We slipped off too many tackles and whilst that has not been the case in previous games, it was today and it is an area we clearly need to get right in two weeks' time," he said.
"In terms of those conditions today I thought there was a period in that second quarter where Fiji handled those moments better than we did, having started the game very well.
"It is clear what we have to get right in two weeks' time against Argentina as we know that is going to be a huge game and one we are going to be focused on right from this minute."
Many pundits felt Borthwick, a 43-year-old former England captain, had been given something of a 'hospital pass' when the Rugby Football Union appointed him head coach in December -- less than a year out from the World Cup -- after sacking Eddie Jones.
The veteran Australian boss, for all he can be a controversial and outspoken figure, has a proven track record in tournament rugby as he demonstrated while overseeing England's run to the World Cup final in Japan four years ago.
But Borthwick insisted England remained a united squad .
"I share the disappointment and frustration," he said.
"We are all in this together. This is a group that is together, this is a group that is tight."
