The Mexican president rode in the state carriage in 2015 but President Obama chose to spare his hosts the security nightmare. Mr Trump faces a more severe threat in October

Donald Trump waving from the Queen’s royal carriage is not a scenario many would have foreseen a year ago, but it has become a very real prospect, forcing security services to plan an unprecedented lockdown.

The White House has made clear it regards the carriage procession down the Mall as an essential element of the itinerary for the visit currently planned for the second week of October, according to officials.

Security sources have warned, however, that the procession will require a “monster” security operation, far greater than for any recent state visit.

President Obama arrived at Buckingham Palace in an armoured motorcade during his 2011 visit LEWIS WHYLD/PRESS ASSOCIATION

Barack Obama chose to spare his hosts the task of securing the journey from Horse Guards Parade to Buckingham Palace when he was welcomed by the Queen in 2011. In a slight change