President Trump and Prime Minister Keir Starmer of Britain said on Monday that they had finalized their trade agreement to lower tariffs on British cars, steel and aluminum, and aerospace equipment, making good on parts of a deal they struck last month to lower barriers on some British goods.

In an executive order issued moments after the two leaders made the announcement at the Group of 7 summit in Canada, Mr. Trump lowered the U.S. tariff on British cars to 10 percent, from 25 percent, and ordered that the United States let a certain amount of British steel and aluminum into the U.S. tariff-free.

The order also lowers tariffs on some airplane parts, which British officials say will benefit British aerospace manufacturers.

“We just signed it and it’s done,” Mr. Trump told reporters. He praised Mr. Starmer’s negotiating skills, saying, “He’s done a very, very good job. You know, he’s done what other people — they’ve been talking about this deal for six years, and he’s done what they haven’t been able to do.”

Mr. Starmer, standing next to Mr. Trump, said that the agreement “implements on car tariffs and aerospace, and now a really important agreement, and so this is a very good day.”

In a brief statement, the White House said that the Commerce Department will still need to set a quota for the volume of British steel that will be able to come into the country tariff-free under the agreement the countries reached in May. Asked by a reporter on Monday if U.S. tariffs on British steel would go to zero, Mr. Trump responded, “We’re going to let you have that information in a little while.”