WASHINGTON, Feb 10 (Reuters) - President Donald Trump moved to substantially raise tariffs on steel and aluminum imports on Monday, cancelling exemptions and duty-free quotas for major suppliers Canada, Mexico, Brazil and other countries in a move that could boost the risk of a multi-front trade war.

Trump signed proclamations that raised the tariff rate on aluminium imports to 25% from the previous 10% that he imposed in 2018 to aid the struggling sector. His action reinstates a 25% tariff on millions of tons of steel imports and aluminum imports that had been entering the U.S. duty free under quota deals, exemptions and thousands of product exclusions.

The proclamations were extensions of Trump's 2018 Section 232 national security tariffs to protect steel and aluminum makers. A White House official said the exemptions had eroded the effectiveness of these measures.

Trump also will impose a new North American standard requiring steel imports to be "melted and poured" and aluminum to be "smelted and cast" in the region to curb imports of minimally processed Chinese steel into the U.S.