The unknown buyer has been building up the dominant position since at least last week, putting a squeeze on the market.
According to the rules of the London Metal Exchange, the trader must lend out copper if it holds between 50pc and 80pc of the total to maintain day-to-day liquidity in the market. The trader is currently lending at a 0.5pc premium to the cash price.
The premium for spot price copper over delivery in three months' time reached $89 in the middle of this week - the highest in two years.
Stockpiles in London have fallen by more than a third since their levels at the beginning of the year.
LME copper was steady at $8,720 per tonne this morning, having reached a high of $8,732 earlier. A record price of $8,966 was hit in mid-Novermber.
The large position is not the only reason the copper price is high.
There are fears of a shortfall in supply next year, as mining production is not expected to keep pace with rebounding demand following the recession.
Two US investment banks and one UK company also want to launch exchange-traded funds linked to copper, which is likely to suck demand out of the market further.