Builders came across the device on a site close to Fountain Studios, which also houses The X Factor U.K., in the Wembley area of the British capital, and police ordered an immediate evacuation.

Bomb disposal experts swarmed the scene on Thursday night to work on removing the nine stone (50 kilogram) explosive, which could take up to a week.

Insiders revealed that Britain's Got Talent founder Simon Cowell and TV bosses are now worried the show's live semi-finals, which are due to air in the U.K. on Monday (25May15), could be hit by the removal operation, and they are trying to find an alternate filming location.

A source tells Britain's The Sun newspaper, "We are now in an absolute panic about Monday. If the area does take a week to secure, then we won't be able to hold the semis at Fountain. We've been holding BGT (Britain's Got Talent) and X Factor live shows there for years and there aren't many other venues in London suitable at such ridiculously short notice...

"It was all incredibly dramatic and scary. Police came into reception and ordered everyone out immediately. No one was allowed back in and lots of people's cars have been left stranded behind the police tape."