LONDON (Reuters) -Prices of aluminium premium contracts on Comex exchange beyond August have fallen due to some speculation that tariffs on U.S. imports of the metal could be halved or that top supplier Canada could get an exemption.

Taxes on aluminium are part of a broader U.S. effort to revive domestic smelting capacity and cut reliance on imports of the metal used in the transport, packaging, power and construction industries.

Aluminium shipped to the United States has been subject to a 50% levy since June 4. The physical price premium for the August contract has climbed above 70 U.S. cents a lb or $1,543 a metric ton, up nearly 90% since the end of May.

U.S. buyers of aluminium on the physical market typically pay the London Metal Exchange benchmark plus the Midwest premium to cover costs including freight and taxes.

Traders and industry sources said the premium on Comex, part of the CME Group, for September onwards had dropped because of bets on lower prices due to some expecting President Donald Trump to row back on aluminium import taxes.

The Midwest aluminium premium for September is currently trading around 67 cents a lb and at 60 cents a lb for December.

Numbers from information provider Trade Data Monitor show the United States imported more than 3.9 million tons of aluminium last year of which 70% or more than 2.7 million tons were shipped from Canada.

However, consultancy Harbor Aluminum expects tariffs on U.S. aluminium imports to stay because the U.S. government believes that tariffs are important for national security.

"Harbor does not see the U.S. reducing the 50% tariff or offering exemptions ... more so now that it has become 'addicted' to the revenue the tariffs provide," the consultancy said in a note.

"We remain bullish and continue to expect the Midwest premium to trend toward 78 cents a lb before the end of the year and 87 cent a lb in 2026."