British supermarkets have admitted that they could be selling olive oil produced in a way which kills millions of songbirds every year, as they promise to investigate their sourcing.

Many harvesters across Italy, Spain and France suck olives from trees using machines, and do this at night, which means sleeping birds who think they have found sanctuary in the olive branches are dazzled by the bright lights and sucked to their deaths.

These companies harvest during the evening because doing so is believed to preserve the aroma of the olives, due to the cooler air temperatures.

Birds including robins, goldfinches, greenfinches, warblers and wagtails are among the worst affected during the harvest season which is between October and January. Findings in the journal Nature suggested that over two million birds were killed in Spain alone in a year.

An investigation by The Telegraph found that British supermarket shelves are likely stocking olive oils produced in this ecologically destructive method.

Supermarkets including Waitrose, Tesco and Sainsbury's could not confirm whether the brands they stock are machine harvested at night.

A Tesco spokesperson said the supermarket is investigating how it will pick olives for its oils this year after concerns were raised by shoppers.