While Mr Ombeta says the letter is genuine, sources at City Hall have been reluctant to confirm its details.
Questions linger over the nomination letter purporting that Nairobi Governor Mike Sonko has appointed political rival and Kenyan deportee Miguna Miguna even as Nairobi County Speaker falls short of confirming or denying the authenticity of the viral letter.
In an interview with Hot 96 FM on Thursday morning, the Nairobi County Assembly Speaker Beatrice Elachi said the signature in the viral letter was Mr Sonko's but she had not yet received the letter.
"The Constitution is clear that a deputy governor needs to be a Kenyan citizen. I would tell Miguna to first clear his issues with the Kenyan government. He is Canadian," said Speaker Elachi.
Mr Miguna's lawyer Cliff Ombeta on Wednesday said that Mr Sonko has settled on the embattled opposition politician as his deputy, a surprise announcement that was greeted with disbelief since Mr Miguna was one of his fiercest critics in the race for the Nairobi governor's seat.
Nairobi County Deputy Communication Director Elkana Jacob shared the letter on his Facebook page but it was greeted with doubt with many City Hall sources reluctant to confirm its details.
However, Mr Miguna has not yet confirmed or denied the appointment.
The county boss has been under pressure to nominate a deputy since the departure of Polycarp Igathe in January this year.
Mr Ombeta said Mr Miguna's name has been forwarded to Speaker Elachi for vetting and approval.