"I think the term "single market" is increasingly useless", Johnson said.
"Clearly when the government and parliament considered this before, there were concerns about a range of action".
There has been much speculation over the UK's trading status once the country leaves the European Union, with other member states' leaders saying no country can be a member of the single market unless all European Union citizens have the right to live and work in that country.
The Prime Minister's spokeswoman added: "As the Foreign Secretary has said, lots of the options that people are looking at have got hard issues that would need to be addressed".
The British business community is becoming increasingly anxious that the government will push for a so-called "hard Brexit", involving leaving the single market and potentially even reverting to World Trade Organization (WTO) rules to trade with the rest of Europe.
Johnson, a leading figure of the successful campaign for Britain to leave the European Union, is one of a trio of Eurosceptics at the heart of May's government who will have a key role in shaping the country's new approach to global trade and diplomacy.
He also said: "Those who prophesied doom have been proved wrong and will continue to be proved wrong".
Archbishop told in 5 am call he'll be cardinal
In a major surprise, he gave the top slot to the courageous Italian nuncio in Damascus, Syria, Archbishop Mario Zenari. He met the pope when Francis visited vehicle at the end of 2015, a trip he was instrumental in organising.
"We are ready to ensure the safe withdrawal of armed rebels, the unimpeded passage of civilians to and from eastern Aleppo, as well as the delivery of humanitarian aid there", Russian Lieutenant General Sergei Rudskoy said in a televised briefing. "But Brexit is not any sort of mandate for this country to turn in on itself and haul up the drawbridge or to detach itself from the worldwide community".
Asked whether he backed the move, Mr Johnson told the Commons Foreign Affairs Committee: "It is not a Government priority, I must tell you regretfully".
He also warned European countries not to "punish United Kingdom financial services" for Brexit, as this made no "economic sense" for them.
The British government lost a 2013 parliamentary vote over plans to bomb the forces of Syrian President Bashar al Assad, but has been involved in bombing raids against Islamic State in Syria since winning the support of lawmakers last December. Moscow accused him of "Russophobic hysteria".
"Most people, I think, are now changing their minds about this and they are thinking "We can't let this go on forever, we can't just see Aleppo pulverised in this way, we have to do something", he said.
Russian Federation denies attacking civilian targets in Aleppo, including an air raid on a United Nations aid convoy last month for which it has been widely blamed.