One of the country's most prestigious stores has pledged to investigate claims of animal mistreatment at farms linked to a company that has supplied it with foie gras.

Activists from Peta, People for the Ethical Treatment of Animals, filmed undercover at two French geese farms that supply the firm that has sold the delicacy to Fortnum & Mason in Piccadilly, London.

The two farms belong to a co-operative that uses an abattoir where Peta investigators also filmed. Peta claims the footage, seen by the Observer and which Fortnum's is reviewing, suggests some of the birds were killed without being stunned first, in contravention of French law. The footage also suggests that birds are force-fed to the point of being unable to move freely.

Fortnum & Mason declined to identify the two farms belonging to the co-operative, which it says supply its own brand of foie gras, for fear of reprisals from animal rights activists, but it insists it visits them regularly to ensure its high standards of welfare are met.

However, the store has until recently sold another brand of foie gras that, according to a representative from the co-operative, captured on video by Peta, can come from any of the 24 farms. It appears that the store, which boasts two royal warrants, removed the brand from its shelves when it became aware Peta had infiltrated the farms.

One of the farms infiltrated by Peta featured in a promotional video in which Fortnum's previous managing director, Beverley Aspinall, highlighted the store's stringent welfare standards. Footage taken at the farm shows blood dripping from an injured bird.

The Peta footage also appears to show a bird at another farm unable to stand as a result of force-feeding, contrary to assurances provided in Fortnums' corporate social responsibility policy.

According to its website, Fortnum's insists "we will never allow our birds to reach a stage where they can't support their own weight and move around freely".

Footage from the abattoir appears to show that some geese are not properly stunned and continue to lift their heads for some time after their throats are cut. French regulations state that "the stunning of animals before slaughter is mandatory before killing".

Peta intends to release the footage on the internet with a voiceover from Sir Roger Moore, a critic of foie gras production. "We are imploring all Fortnum & Mason customers to keep out of the store until it joins Selfridges, Harvey Nichols, Waitrose and others in refusing to sell foie gras, or 'torture in a tin'," a Peta spokesman said.

The row between Peta and Fortnum's is the latest battle in a long campaign by the animal rights group to stop the store selling foie gras. The product, which is produced by force-feeding geese so their livers expand, is prized by gourmets.

However, in blogs several gourmets have defended the store's right to sell the delicacy and say it has gone to great lengths to ensure the food is produced to the most humane standards.

Peta's campaign against Fortnum's comes at a time when foie gras production is under scrutiny. California has banned the sale of foie gras, while the UK catering company Compass Group, which supplies Michelin-star restaurants and Whitehall departments, has dropped the product from its menus.

Last year, in response to complaints from Peta about its sale of foie gras, Fortnum's was ordered by Westminster trading standards to amend misleading statements in its corporate social responsibility that suggested all of its meat met the highest welfare standards.

A Fortnum's spokesman said: "Fortnum & Mason maintains rigorous quality control procedures and would expect all of its suppliers to adhere to our gold standard production requirements."

The spokesman stressed it had not yet been able to prove or disprove Peta's allegations. "We are acutely aware of our reputation as one of the world's leading purveyors of quality foods and have subsequently instructed the appropriate individuals within the business to look into the allegations.

"However, it's important we do not pre-empt that process in light of the questions that we continue to have regarding the footage in question."