Mexico’s leader said any change to a trade pact between the three biggest north American economies would need input from all the nations, after a report that the U.K. was weighing joining the accord. 

“All of the actors have to look at this,” President Andres Manuel Lopez Obrador said Thursday. “We cannot jump ahead,” he said, adding that the agreement between the U.S., Canada and Mexico was “such an important achievement.” 

A person familiar with the U.K.’s thinking said Tuesday that London is exploring its options to join the trilateral trade agreement known as the USMCA, a recognition that the Biden administration won’t start talks on a bespoke bilateral deal with the U.K. -- billed as one of the prizes of Brexit -- any time soon. 

Any U.K. effort to join USMCA would face major political and procedural obstacles and could be interpreted as an effort by Downing Street to play catch-up given the EU has separate trade deals with Canada and Mexico. 

USMCA is the successor to the North American Free Trade Agreement, which the Trump administration rebranded after two years of tough negotiations with its southern and northern neighbors. 

Trade observers also noted that any attempt to join could take years of wrangling and put U.K. negotiators in a weak position, at the mercy of three political systems that might not see much incentive to welcome another member -- particularly one with an economy about the size of California’s.

Canada’s foreign affairs department and the office of the U.S Trade Representative both said they hadn’t received requests from the U.K. to accede to the pact, adding that it doesn’t have mechanisms allowing other countries to join in its current form. 

Mexican Economy Minister Tatiana Clouthier on Thursday met U.K. Foreign Minister Liz Truss, who until last week was the nation’s chief of trade. In a tweet, Clouthier said Mexico was committed to firming up a U.K.-Mexico trade deal.

“We’re still keen to agree a U.S.-U.K. trade deal -- that remains a priority,” said Max Blain, a spokesman for Prime Minister Boris Johnson, who met with President Joe Biden in Washington this week. “What we’re focused on is a U.S. deal and there are no plans to go beyond that.” 

Global Affairs Canada, the nation’s foreign-relations department, said it looks forward to continuing its work toward a future bilateral trade agreement with the U.K. that will benefit people and businesses on both sides of the Atlantic.