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Speaking at the conservative Heritage Foundation, Fox stopped short of labelling US President Donald Trump a protectionist.
Fox said London is preparing the way to launch formal talks with Washignton by the March 29 Brexit date, including meetings with US Trade Representative Robert Lighthizer and Commerce Secretary Wilbur Ross.
Preparations are starting now because of legal requirements in each country, including the need for a public comment period in Britain, and the 90-day notice the US administration must provide to Congress before entering into free trade talks.
There have been "very constructive ongoing discussions on a number of fronts," Fox said, adding that the discussions will depend on the post-Brexit relationship with the EU.
While in London earlier this month, Trump warned Prime Minister Theresa May that she must get a "carve out" from the EU to allow a bilateral free trade deal with the United States.
Asked if he considered Trump a protectionist, Fox said, "The United States has always been a free trading country." But "it's no secret we have differences" about the national security mechanism used to justify the US tariffs actions.
Britain and the EU, like other US allies, have been hit by the biting Trump administration tariffs on steel and aluminum, and retaliated with punitive duties on key US products like bourbon, blue jeans and motorcycles.
Trump was due to meet later Wednesday with European Commission chief Jean-Claude Juncker for what is set to be a tense effort to resolve a festering trade dispute between the two key economies.
Fox said the dispute and White House meeting demonstrated why Britain was right to leave the EU: so the country can have an independent trade policy and not be represented "by bureaucrats in Brussels."
"We want to see de-escalation of tension over tariffs... so we can get back to real issues," he said.
In his speech, Fox strongly defended global free trade as something of "mutual benefit" as well as a force that cannot be stopped.
While there is growing concern in rich nations about the impact globalisation, "The case against protectionism is as close to settled science as anything ever will be." Governments can better serve their workforce "not by turning our back on free trade but providing mitigation" like retraining for workers.
In addition, the WTO rules governing trade need to be modernized, and made "robust and enforceable."
WTO Director-General Roberto Azevedo agreed, saying in comments on CNBC that "we need to keep reforming and updating the system."
And despite withering criticism from Trump that the WTO is unfair to the United States, Acevedo said there is no indication Washington intends to pull out.