Alonso said: "We were not competitive all weekend and it's not normal to see Ferrari out of Q3. We need to recover now and it was a bad Saturday.
"We didn't improve the car enough in the last four to five races and we ordered some new parts, but they didn't do what we expected. We need to keep on working…"
Alonso's 10th place performance was his worst of the season and followed two sixth places in qualifying in Monaco and Canada. Previously, he had two fifth and three thirds.
To make matters worse for Ferrari, Alonso's teammate Brazilian Felipe Massa continued to be dogged by misfortune.
After big accidents in Monte Carlo and Montreal, he had another 'off' on Friday and on Saturday qualified only 12th with an almost-desperate effort to avoid further humiliation on a circuit where Ferrari normally perform strongly.
Alonso came to Silverstone as one of only two drivers to have won twice before at the track, but his hopes appeared to evaporate as Ferrari battled in search of a competitive set-up.
"We must keep working, more and more," he reiterated, his newly-created 'goatee' beard having failed to deliver any luck to balance out the team's lack of performance.