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After accepting she had failed to deliver Brexit, May said on Friday she would resign on June 7 and trigger a contest within the Conservative Party to replace her.
Some contenders have said they plan to renegotiate her deal with Brussels.
"The legal text that was agreed at the council with the prime minister was that we would not be seeking to reopen the Withdrawal Agreement, that was part of the process under which the extension was approved," the spokesman said.
"But ... I am not going to be commenting on the positions others set out as part of the leadership contest.
As the prime minister said on Friday ... it is now for others to find a way forward."