The Verge news tech site revealed the issue after it noticed a window reflection revealed a cameraman holding what appeared to be an SLR camera.
In a blog post the handset maker said it should have "posted a disclaimer".
The news threatens to take the shine off the launch of the Lumia 920 phone.
Investors had already shown doubts about whether the Windows Phone 8 handset could revive the Finnish company's fortunes, sending its shares nearly 8% lower on Wednesday.
In Thursday's trade the stock fell a further 6%.
The advert showed a man filming his girlfriend using the new smartphone while both of them were riding bicycles.
The footage cut from professionally-filmed material, featuring both actors, to what looked like it was the video captured by the man's Nokia device.
The footage was intended to support the firm's claim that its technology helped reduce image blur. It was shown at a press conference on Wednesday to support Nokia's boast that the Lumia 920 featured the best smartphone image quality on the market.
The Verge's article said: "too bad it's faked".