Two senior Barclays Capital
The traders were named as Iain Macrae, head of the metals desk at Barclays, and Christian Saunders. The two traders could not be reached for comment.
Barclays would not specify the size of the loss but said it was not big and that there was no abnormal trade, meaning that no rogue positions were taken.
Metals dealers said Barclays was long copper in the summer on outright prices, spreads and options when concerns about euro zone debt hit risk markets globally and copper fell by a third.
It was a triple whammy, said a London metals dealer.
LME copper tumbled by a third from $9,900 (6,316.60 pounds) a tonne at the start of August to $6,635 at the end of September.
Barclays has been bullish on copper this year. In a research report released in mid-July it forecast copper would climb from $9,137 a tonne in the second quarter to $12,000 a tonne on average in the fourth quarter. The fourth quarter average to date for copper is $7,530.
Barclays also went long Dec 2012 versus Dec 2013 copper, betting on a widening premium, but the spread narrowed significantly at the same time outright prices fell in September.
Compounding those losses, a trade on the February to March 2012 aluminium spread unravelled this week.
In aluminium, several investment banks including Barclays, in September began building large long aluminium positions into early 2012.
The banks were said by traders to be targeting those who have long-term financing deals on metal and store it in warehouses. Owners of metal in warehouses commonly hedge their positions by taking short positions on the LME.
Since September, traders said Barclays had bid the February 2012 contract for aluminium up to a premium against March, but it dropped to a steep discount this week and Barclays was forced to liquidate the position.
The Feb-March spread traded as high as a $9-a-tonne premium on December 5 and closed on December 21 at a discount of $11.50 a tonne.
We knew they were liquidating their nasty position, said an LME metals trader.
Barclays shut its prop trading arm this year as it streamlined operations and pared back in areas under most regulatory pressure, alongside the exit of star commodities trader Todd Edgar and several colleagues.
(Reporting by Melanie Burton. additional reporting by Josephine Mason, Anirban Nag; Editing by Richard Mably)