Alonso led for much of the three-and-a-half hour session as he ousted Mercedes development driver Esteban Ocon from the top spot early in the second hour and stayed there until the chequered flag with a succession of improvements.

The two-time champion's benchmark time of 1m31.290s came during the third hour of running, putting him 1.543s clear of Ocon on the Frenchman's first appearance for Mercedes.

Ocon was, however, the most productive driver of the morning, clocking up 80 laps of the Northamptonshire circuit, three more than nearest rival Carlos Sainz.

Third quickest was Alex Lynn in the Williams, the GP2 driver lapping just over three seconds shy of Alonso's pace and just 0.013s quicker than Charles Leclerc in the Ferrari.

The Monegasque had been down in ninth until a series of late hot laps, which narrowly demoted Force India's Nikita Mazepin to fifth on his first morning in Grand Prix machinery.

Sainz completed the top half-dozen for Toro Rosso, ahead of Santino Ferrucci, who made his F1 test debut for Haas, and Pascal Wehrlein, who is performing development work for Pirelli using a 2014-spec Mercedes.

Completing the timesheets were Manor's Rio Haryanto, Renault's Sergey Sirotkin and Pierre Gasly, whose Red Bull was equipped with a Halo cockpit protection device for his installation laps.

The Frenchman completed the fewest number of laps of the morning, seven, 10 fewer than Leclerc.

Morning testing times: