Although the incident broke the car’s front-wind endplate, Alonso kept it largely under control and out of the barriers as it slid wildly up the kerb on the outside of the circuit.
Aston Martin said the incident was not caused by driver error, adding the car is difficult to drive and the brakes locked.
Team representative Pedro de la Rosa said: “What we have right now is a very difficult car and drivers doing their best and doing an incredible job to drive the car in a reliable and safe way.”
The Ferrari drivers were quick throughout, but the car looked on edge.
Both made at least two visits to escape roads at the first corner and chicane, and Leclerc kept the car at Sainte Devote as he suffered a big oversteer slide as he sought to put the power down to launch it up the hill.
McLaren’s Lando Norris was sixth fastest, split from team-mate Oscar Piastri by Audi’s Nico Hulkenberg.
The second Audi of Gabriel Bortoleto was ninth, ahead of the Alpine of Pierre Gasly and the Williams of Alex Albon and Carlos Sainz.
