Tuilagi, whose elder brother Alesana was sanctioned for the same offence last week, wore the Opro mouthguard in England's first two victories over Argentina and Georgia.
A Rugby Football Union spokesman said: "Manu wore a mouthguard where the branding was visible in the Georgia match when it shouldn't have been so therefore he's been fined NZ,000.
"It was a genuine error and he's trying now to focus on Saturday's game."
The RFU spokesman insisted England had not been fined for any other breaches during the tournament.
The fact both brothers were fined for the same offence has led to suggestions they deliberately flouted the IRB's strict advertising and sponsorship rules.
But Tuilagi insisted they were not part of any ambush marketing campaign.
"I didn't know I had worn the wrong gumshield and I got fined for it," Tuilagi said.
"We always wore the same mouthguard, mine is the England one from the under-18s but it's got 'Opro' on the front and that's not allowed in the World Cup.
"I coloured it in with marker pen for the Romania game. I wore it against Argentina but they didn't know."
Tuilagi's fine is England's latest brush with authority on a World Cup campaign being dogged by controversy.
Last week, Martin Johnson was forced to suspend two members of his coaching staff after they were spotted illegally changing the ball during England's win over Romania.
The RFU, who had feared a potential points deduction which could have wrecked England's title bid, banned kicking coach Dave Alred and fitness specialist Paul Stridgeon from the Scotland game.
Johnson was forced to angrily reprimand James Haskell, Dylan Hartley and Chris Ashton for their reportedly teasing a hotel worker with lewd comments.
The England manager has defended Mike Tindall, who has reportedly "apologised unreservedly" for misleading the RFU about his whereabouts on a now infamous night out in Queenstown.
On the field, England have had Courtney Lawes and Delon Armitage suspended.
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