US war hawk Bolton to meet with Boris Johnson
The National Security Advisor is the most senior US official to meet with Johnson since he succeeded Theresa May as prime minister last month.
The Trump administration has been pursuing a "maximum pressure" campaign designed to force the Islamic republic to limit nuclear and military activities.
As part of that approach, the US has reupped sanctions on the country, boosted its own troop presence in the Gulf region, and reaffirmed its alliance with Saudi Arabia and the UAE - arch enemies of Iran who have butted heads with their Gulf neighbour over the crucial Strait of Hormuz and the Islamic republic's links to the Yemeni Houthi rebels.
The UK was also directly drawn into the Gulf crisis when the British Navy in July seized an Iranian tanker suspected of carrying oil bound for Syria, in violation of EU sanctions, off the coast of Gibraltar.
That incident sparked a series of ongoing tanker seizures in the Gulf.
According to Spanish newspaper El Pais, Bolton had "engineered" the Gibraltar tanker incident, utilising US intelligence to tip off the UK in the hopes of securing the tanker's seizure.
Bolton is also set to pressure London to drop - or at the very least severely restrict - plans to use 5G technology made by China's Huawei when it rolls out the next-generation data network.
Washington was not expecting a decision from London on either matter during the two-day visit, US media reported.
The Downing Street spokesman said the UK's position on both Iran and Huawei "remains the same".