Barker, Olympic champion in the team pursuit in Rio, appeared to be well-placed to succeed absent team-mate Laura Kenny as champion in the non-Olympic scratch race.
The 22-year-old Cardiff racer was one of six riders to lap the field with seven laps of the 40-lap race to go and launched her sprint with just under two laps to go. She tried to cling on, but Italy’s Rachele Barbieri narrowly beat her to the line to claim the world champion’s rainbow jersey.
“I kind of wish I’d lost by a little bit of a bigger margin,” said Barker, twice a team pursuit world champion and 2012 junior road time-trial world champion.
“I feel like I was good enough to win it. I know I’m going to watch it back and pick it apart and think, ‘Argh, that’s where I lost the race’.
“I am happy with a silver medal and quite proud of it, but I’m not going to get to wear the world stripes next time I race the scratch race, which is what I wanted.”
Four-time Olympic champion Kenny, who married six-time Olympic champion Jason Kenny last September, won the title last year, but is now pregnant and taking a break.
Belgium’s Jolien D’hoore, who was third, told Barker prior to the podium presentation that she “looked just like Laura” on the track.
That might have been a comment on the Britons’ similar physical stature, but also a compliment in the way Barker animated the race.
She tried to persuade Holland’s Kirsten Wild to lead the sprint and she would then have tried to accelerate from behind her, but the Dutchwoman did not want to give Barker the initiative.
And having contributed little to the group which claimed the lap, Barbieri summoned the energy to win the dash for the line.
Barker has the points race to come on Sunday’s final day of competition and the Madison on Saturday’s fourth day, where she is slated to ride with Emily Nelson.
Another opportunity for a medal comes on the second day, when the women’s team pursuit concludes and Barker is expected to come into the line-up.
None of the Olympic gold medal-winning quartet took part in qualifying. Barker was conserving energy for the scratch race, while Katie Archibald is focusing on individual events here. Joanna Rowsell-Shand has now retired and there is also Kenny.
A new-look quartet of Ellie Dickinson, Emily Kay, Manon Lloyd and Nelson qualified fifth in four minutes 21.449 seconds.
Only the top four-ranked teams have a chance to make the gold medal ride-off, but Britain will hope to be one of the two fastest losers to contest the race for bronze.
Britain reached a ride-off for bronze with Italy in the corresponding men’s event after also finishing fifth in qualifying. Two-time Olympic champion Steven Burke and Kian Emadi swapped out between the afternoon qualifying session and the first round, when Mark Stewart and Chris Latham replaced them.
Stewart, Latham, Andy Tennant and Ollie Wood combined to finish more than two seconds quicker than earlier in the day in 3:56.796.
The final will be contested by New Zealand and Australia.