How low can they go? As economic gloom sets in, that question is increasingly dominating the minds of metals investors.

Industrial metals such as copper, aluminium and nickel are sensitive to changes in the global economic cycle, because of their uses in construction and manufacturing. So, as expectations for economic growth in Europe, China and the US have been scaled down, prices have fallen to the lowest in years.

Aluminium, used to build aircraft, cars and to make fizzy drinks cans, has tumbled 20 per cent since March and at $1,899 a tonne on Monday was close to its lowest since late 2009. Nickel, used to make stainless steel, has fallen 30 per cent since February to touch a three-year low of $15,450 a tonne. Even iron ore, a favourite of commodity bulls, has fallen 22 per cent since April.