Liam Fox has admitted it would be "optimistic" to expect a UK-EU trade deal to be complete by March 2019.
The International Trade Secretary said recent experience suggested it would take longer than the two-year Article 50 period to negotiate the future economic relationship with the bloc.
Instead he envisaged an "implementation phase" where the UK was technically outside the EU but continued to abide by its rules on a voluntary basis.
Dr Fox is currently in the US laying the groundwork for a future US-UK post-Brexit trade deal.
In a Q&A after a speech at the American Enterprise Institute, he said: "There’s a growing consensus amongst the Cabinet that we will leave the European Union but we will have a transition and implementation phase where we’re outside European law but voluntarily would choose to keep a number of rules as part of the acquis to give our businesses in particular and our inward investors the stability to understand what the new environment is going to look like.
"Because it would be nice to think we could get a full free trade agreement by the time we get to March 2019, but that would be an optimistic view of recent free trade agreements."
He also hit out at the British media over what he claimed was an "obsession" with chlorine-soaked chickens.
The question of food hygiene standards has come to the fore after claims that the UK could be forced to accept imports of US poultry washed in the chemical.
The practice is banned under EU regulations, but Downing St has not ruled out changing the standards once the UK has left the bloc.
At a media briefing this morning, Theresa May’s spokesman would not categorically rule out a change.
"Our position when it comes to food is that maintaining safety and public confidence in the food we eat is the highest priority and any future trade deal must work for UK farmers, businesses and consumers," he said.
Dr Fox gave short shrift to a reporter asking whether he would personally be prepared to eat chicken that had been treated with chlorine.
He said: "In a debate which should be about how we make our contribution to global liberalisation and the increased prosperity of the UK, the US and our trading partners, the complexities of those, the continuity agreements, the short-term gains that we may make, the opportunities that we have and our ability to work jointly towards both a free trade agreement and WTO liberalisation, the British media are obsessed with chlorine-washed chickens, a detail of the very end stage of one sector of a potential free trade agreement. I say no more than that."