Boris Johnson, the foreign secretary, arrives in Downing Street Leon Neal/Getty Images

LONDON — Boris Johnson has heaped more pressure on Prime Minister Theresa May to negotiate a short transition period after the UK's withdrawal from the European Union as he endorsed a new think tank which supports a hard Brexit.

The foreign secretary's comments are widely seen as a challenge to May, who announced last week in her Florence speech that the government would aim for a two-year implementation period.

Johnson was speaking at the launch of the Institute for Free Trade on Wednesday evening, a privately funded think tank, which was hosted by the Foreign Office.

The think tank advocates "unilateral free trade" which would mean dropping tariffs on imports into the UK in order to achieve "unrestricted commerce."

Johnson said: "When you consider what we have done in the past, you can imagine what our brilliant companies are going to be able to do when they are finally unbound, unshackled, unleashed from the coils and toils of the common commercial policy.

"We have an extraordinary future ahead of us. Let’s hope the date is soon upon us without too long a transition period, n’est pas?"

Johnson's fresh challenge comes just days before the Conservative Party conference in Manchester, where tensions are expected to be high as the cabinet remain split over how Brexit should progress.

It was reported on Monday that Chancellor Philip Hammond supports a lengthy transition period. Allies of the chancellor said Johnson's approach to Brexit is "simple-minded."

The foreign secretary previously published a 4,000-word article which laid out his vision for a "glorious" Brexit, but this new intervention is a further provocation after May's speech attempted to unify the cabinet.

The Institute for Free Trade's president, Eurosceptic Conservative MEP Daniel Hannan, called on the government to embrace a Singapore-style free trade model after Brexit.

He said: "I’m looking at [the] high commissioner of Singapore [in the front row]. They have gone from being half as rich as us to twice as rich as us. What was the magic formula? Just do it. They dropped their barriers."

The prime minister has attempted to make it clear that the UK is not seeking to undercut the single market and become an offshore haven like Singapore following Brexit.

Johnson was joined by International Trade Secretary Liam Fox and Environment Secretary Michael Gove at the launch, both of whom are prominent hard Brexit campaigners.

May will be embarrassed by the level of cabinet support for the new project, especially as it comes just a week after she hoped to have calmed down cross-party arguments with her Florence speech.

Fox said: "We may think the benefits of free trade are self-evident but we need to sell benefits to the public. We have to go beyond the economics into the moral argument. As we leave the EU and take our independent seat at the WTO we will champion free trade."

Hannan claimed the UK should use Brexit to revolutionise its economic policy. "It’s not every day a country of our size gets to draw up a new trade policy from scratch," he said.

Labour MP Pat McFadden, who is a leading supporter of the Open Britain campaign said: "Any Prime Minister with an ounce of strength would not permit her Cabinet colleagues to launch think tanks undermining the Government’s policy, let alone in a Government building. But this just shows how weak Theresa May’s position has become.

"The Florence truce has not lasted long. Ideology over Europe has divided them before and it’s doing so today. But the key point is that this is not a game. The country’s future is at stake. And the economic interests of the country must come before the nationalist ideology to which too many Ministers subscribe."