April 24 (Reuters) - Valero Energy VLO.N reported a first-quarter loss compared with a year-ago profit on Thursday, weighed down by lower refining margins during a quarter marked by elevated maintenance and turnaround activity across the U.S. refining sector.
Valero, which kicked off refiners' earnings, said net loss attributable to stockholders was $595 million, or $1.90 per share, in the three months ended March 31, compared with last year's profit of $1.2 billion, or $3.75 per share.