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The Australian led the early laps before dropping behind Arvid Lindblad, who went on to dominate the race.
Starting from pole, Mansell held the lead away from the start, sweeping to the inside to protect his position into the opening corner.
He headed Lindblad, Nikola Tsolov and Luke Browning through the opening sequence of corners and held a 1.001s lead at the end of the opening lap.
Lindblad had clawed his way back into DRS range by the end of Lap 2, allowing the Prema driver the opportunity to open the rear wing and close on the race leader ahead.
He wasn’t close enough at that point to mount a challenge but was as they started Lap 4. That saw Mansell go defensive into Turn 1, holding the lead over his Red Bull-backed rival.
Next time around, Lindblad attacked again, making it stick around the outside into Turn 1 to claim the lead and quickly extend a small gap.
Behind the front two, Browning had improved to third over Tsolov as the top 16 ran in a DRS train.
As they started Lap 9, Lindblad had moved out of DRS range of Mansell with a 1.092s lead as he flashed across the line.
The second-placed Australian was also in clear air, having moved clear of DRS range of Browning in third.
However, the Brit closed that gap and began to challenge for second as the pack started Lap 11.
Lindblad had escaped to the tune of 1.7s out front, Mansell unable to match his pace at that stage of the race.
Browning was unable to make an impact and remained third, dropping back out of DRS range just two laps later.
After 13 of the 25 laps, Lindblad held a 2.025s lead over Mansell, who was 1.184s clear of Browning. Tsolov in fourth remained in contact, as did Leonardo Fornaroli in fifth.
Mansell soon reported spots of rain in the final sector of the lap, his ART team preparing a set of wet tyres in anticipation.
Lindblad continued to move away out front, extending his lead to three seconds after 16 laps, running around 0.3s per lap faster than Mansell behind.
His lead extended to over six seconds at times, though eased back to just over four at the flag as he eased his way to the line.
His margin of victory was 4.5 seconds after 25 laps, with Mansell a comfortable second from Fornaroli, who stole third from Brown as they started the final lap – the chequered flag flying just as rain hit Circuit de Barcelona-Catalunya.
