The pay gap emerged during Variety magazine's talk with producer Suzanne Mackie and fellow executive producer Andy Harries at a media conference in Jerusalem.
They did not reveal either salary - Foy's was put at $40,000 an episode by Variety past year - but told a panel event in Jerusalem that Smith's 2010-2013 starring role on the BBC's "Doctor Who" had been the decisive factor. "However, Hollywood and London may have to reassess whether there should be some kind of SAG pay scale".
She added that many women don't realise how much men get paid, and this may explain the imbalance on The Crown. And while she was perhaps not as well-known, she had starred in popular and respected series before The Crown, including BBC's Little Dorritt.
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Mackie attributed the discrepancy to his experience starring in the iconic British program "Doctor Who" prior to joining the cast of "The Crown".
However, critics say this could only explain the first season - before Foy won a host of awards and nominations for the role.
Mark Wahlberg made $1.5m (£1.1m) but donated it to Time's Up, following news that Williams had received less than $1000 (£720) for her work.
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No salaries have been revealed but Variety reported that Foy's was earning $40,000 (€32,000) an episode previous year.
"The Crown", which costs $7 million an episode to produce, is replacing its leads for the start of filming in July, with Olivia Colman stepping in as the queen and an actor to replace Smith not yet announced. Matt's salary has not been revealed.
Show bosses revealed this week Claire was paid less than Matt - despite being on screen for longer. Yet she still got a smaller paycheck than him for the second season, which earned her another Golden Globe nod.
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And a third fan wrote, "Claire Foy is an incredible actress".