Sony is dismayed that Microsoft is only planning to keep 'Call of Duty' on PlayStation until 2028.

Despite Microsoft Gaming CEO Phil Spencer penning a letter to PlayStation president Jim Ryan pledging to keep 'Call of Duty' on the PlayStation for "several years" to come, Ryan has insisted this is not the case and branded the rival firm's proposal "inadequate on many levels".

He said: "Microsoft has only offered for 'Call Of Duty' to remain on PlayStation for three years after the current agreement between Activision and Sony ends. After almost 20 years of 'Call Of Duty' on PlayStation, their proposal was inadequate on many levels and failed to take account of the impact on our gamers."

Ryan added to GamesIndustry.biz.com: "I hadn’t intended to comment on what I understood to be a private business discussion, but I feel the need to set the record straight because Phil Spencer brought this into the public forum.

"We want to guarantee PlayStation gamers continue to have the highest quality 'Call Of Duty' experience, and Microsoft’s proposal undermines this principle."

The letter was sent earlier this year, with Spencer officially putting the promise in writing.

It comes as regulators expressed their concerns about Microsoft's acquisition of Activision Blizzard.

In a statement to The Verge, Spencer said: "In January, we provided a signed agreement to Sony to guarantee Call of Duty on PlayStation, with feature and content parity, for at least several more years beyond the current Sony contract, an offer that goes well beyond typical gaming industry agreements."

Recently, the UK's Watchdog said Microsoft buying Activision Blizzard could reduce competition.

The Competition and Market Authority raised issues with the tech giant buying the parent company of ‘Overwatch’, ‘Candy Crush’ and ‘Call of Duty’.

In response, Microsoft said they were willing to work with the CMA on the “next steps” to make their deal happen, which would be the largest in their history.

In the ruling, the CMA expressed concerns about the problems it could cause for their competitors, such as Sony.

Sorcha O’Carroll, the CMA’s senior director of mergers, said: "Microsoft could use its control over popular games like Call of Duty and World of Warcraft post-merger to harm rivals.

"If our current concerns are not addressed, we plan to explore this deal in an in-depth Phase 2 investigation to reach a decision that works in the interests of UK gamers and businesses."

Brad Smith, a Microsoft spokesperson said: "We're ready to work with the CMA on next steps and address any of its concerns. 

"Sony, as the industry leader, says it is worried about Call of Duty, but we've said we are committed to making the same game available on the same day on both Xbox and PlayStation. We want people to have more access to games, not less." 

The company argued the purchase is for their expertise within the mobile game sphere and denied prohibiting titles like ‘Call of Duty’ from other consoles, such as Sony’s Playstation.

Saudi Arabia became the first territory to approve the deal between the two companies.