Johnson's comments come days after the U.K.'s Middle East minister Alistair Burt said Britain would not join the U.S.in sanctioning Iran, Reuters reported.
Iran had also warned a year ago that it enjoyed the capability to restore its uranium enrichment operations and even develop its capacity to 100,000 SWUs (Separative Work Units) if the nuclear deal with the world powers failed as a result of the USA non-compliance with its undertakings.
Iraq has stopped trading with Iran in the US dollar after Washington imposed economic sanctions on Tehran, according to the country's central bank on Saturday.
The U.S. sanctions that began last week prohibit any transactions with Iran involving dollars, gold, precious metals, aluminum, steel, commercial passenger aircraft, shipping and Iranian seaports.
US ambassador to the United Kingdom, Robert Wood Johnson, wrote in the British paper the Sunday Telegraph, that "Iran grew bolder" after signing on to the Obama-era deal.
It also buys Iranian-generated electricity to deal with chronic power cuts that have been a key factor sparking mass protests in recent weeks.
More news: US can't be trusted, Iran's Rouhani tells North Korea"Until then, America is turning up the pressure and we want the United Kingdom by our side".
But last week members of the European Union reiterated their desire to remain in the deal and even moved to protect other countries that continue doing business with Iran despite United States sanctions that threatens to punish them. He used the government's Brexit rhetoric to support his case, saying that "global Britain" should use its "considerable diplomatic power and influence" to force a change of approach.
"It is clear that the danger from Iran did not diminish in the wake of the deal", he said.
It saw the regime agree to curb its nuclear programme in return for the lifting of sanctions.
Joining Russia and China, who have thrown their full weight behind the accord, Britain, France and Germany said in a joint statement last week that they "deeply regret" the re-imposition of U.S. sanctions because the Iran deal was "working and delivering on its goal".
"It is time to move on from the flawed 2015 deal", he continued.