"I have determined that imports of aluminium from Canada will no longer threaten to impair the national security, and thus I have decided to reinstate Canada's exclusion from the tariff," the proclamation reads.

While letting Canada out of the national security doghouse for now, the declaration reserves the right to reimpose the 10% tariff, allegedly on national security grounds, if aluminium exports spike again before the end of the year.

The development makes official a commitment by the Office of the United States Trade Representative last month, hours before Canada introduced dollar-for-dollar countermeasures worth C$3.6 billion (US$2.7 billion) on US-made aluminium products.

Imports for the three months to December will be capped at 70,000t, 83,000t and 70,000t, respectively. If actual shipments exceeded 105% of the expected volume for any month during the period, the US will impose the 10% tariff retroactively on all shipments made in that month.

The US also reserves the right to reimpose its 10% duty should imports exceed 105% at any time.

In May last year, the US, Canada and Mexico signed a bilateral deal to lift all retaliatory, or so-called Section 232, tariffs. According to the agreement, if surges in imports of specific steel and aluminium products occur, the US may reimpose Section 232 tariffs on those products. Any retaliation by Canada and Mexico would then be limited to steel and aluminium products.

The US will consult with the Canadian government in December to review the state of the aluminium trade and expected market conditions in 2021.