Scarlett Johansson has quit her ambassador role with Oxfam following criticism over her decision to star in an advertising campaign for SodaStream, the fizzy drinks company which owns a factory in an Israeli settlement in the West Bank.

In a statement, Johansson said she was ending her relationship with the humanitarian group over "a fundamental difference of opinion".

"Scarlett Johansson has respectfully decided to end her ambassador role with Oxfam after eight years," the statement said. "She and Oxfam have a fundamental difference of opinion in regards to the boycott, divestment and sanctions movement. She is very proud of her accomplishments and fundraising efforts during her tenure with Oxfam."

SodaStream has come under fire from pro-Palestinian activists for maintaining a large factory in the West Bank. Oxfam opposes all trade in goods produced by Israeli settlements in the Palestinian territories, saying they are illegal and deny Palestinian rights.

The star of current awards season favourite Her and superhero ensemble The Avengers became global brand ambassador for SodaStream International in January. Activists seized on her appearance in an advert for SodaStream scheduled to air during half-time at this Sunday's NFL Super Bowl, the advertising slot with the largest audience on US television.

Johansson has been quick to defend her association with the company, and said last week she was a "supporter of economic cooperation and social interaction between a democratic Israel and Palestine". But Oxfam had come under increasing pressure to dismiss her from the ambassadorial role, with campaigners claiming the position was untenable. "Oxfam is a human rights organisation. They cannot maintain an ambassador if they are involved in a complicit Israeli company built in a settlement. They can't keep both. You can't maintain something and its contradiction," Omar Barghouti, the founder of the boycott campaign, told the Guardian.

Oxfam said it accepted Johansson's decision to step down and was grateful for her "many" contributions.

In a statement, the charity said: "While Oxfam respects the independence of our ambassadors, Ms Johansson's role promoting the company SodaStream is incompatible with her role as an Oxfam global ambassador. Oxfam believes that businesses, such as SodaStream, that operate in settlements further the ongoing poverty and denial of rights of the Palestinian communities that we work to support. Oxfam is opposed to all trade from Israeli settlements, which are illegal under international law."