The hospital provides a good excuse for someone needing a taxi in a rush. Whitehall sources said Swealmeen had not booked his cab the night before the attack, but had ordered it on the day.

It arrived at Rutland Avenue, where he had set up a bomb-making factory in a rented flat, shortly before 11am and he was driven the short distance to the hospital.

“Swealmeen didn’t book the taxi the night before, and when he got in he asked to be taken quickly to the hospital,” said a source, who suggested the request to travel to the hospital may have been to “obfuscate” his real target.

“He might have deliberately gone somewhere nearby the cathedral,” said the source, adding: “If you think of a destination you can tell a taxi driver to take you to quickly, then a hospital is a good one.”

CCTV footage suggests the taxi drove up to the hospital reception “pretty quickly” before coming to an “abrupt halt”, said sources. David Perry, the driver, who survived the explosion with minor injuries, has given full statements to police.

If the theory is correct, the likelihood is that Swealmeen had planned to blow up military veterans and dignitaries in what will be considered an Islamist attack.

The motive has been obscured because in 2017 he had converted to Christianity, but there are increasing suspicions that his conversion was a sham in an attempt to bolster a failed asylum application.

Police said on Friday that Swealmeen had purchased components for his bomb using a number of aliases. The device – similar but not identical to the bombs used in the 7/7 attacks on London in 2005 – contained ball bearings to act as shrapnel.

Although the detonator exploded, causing the fireball inside the taxi that killed Swealmeen, of Syrian and Iraqi heritage, the main bomb did not go off.