It may not be the infamous millennium bug which spoils the new year's celebrations for millions around the world, an errant flock of birds may be enough to stop the chimes of Big Ben which traditionally mark the start of 1 January.

The clock of the Houses of Parliament has long been considered the epitome of Greenwich Mean Time - the universal standard.

This status has prompted planners to make it the centre of millennium eve celebrations, while its age has convinced them to take special precautions against an untimely breakdown.

A team of engineers will spend the evening in the tower, monitoring the 140-year-old mechanism to ensure that 2000 begins with a bong.

Despite a reputation as one of the world's most accurate mechanical clocks, Big Ben has by no means escaped the odd mishap over the years.

In 1944, a flock of starlings settled on one of the hour hands, their weight halting the powerful mechanism.

Feather weight

This seems unlikely to be repeated at midnight on the 31 December - unless a rather hefty owl chooses to brave the pulsing spotlights which will be trained on the tower.

The weather could be the greatest threat to a successful ringing in of the new year.

Ice and snow slowed the clock by an unforgivable 10 minutes on New Year's Eve 1962. Thankfully thick fog masked the embarrassing discrepancy until workmen could defrost it.

Michael McGann, the Keeper of the Great Clock, has few fears that Big Ben will experience a technical failure. Despite its age it regularly chimes to within two seconds of the speaking clock.

This incredible reliability is largely thanks to its designer, the lawyer and amateur horologist Edward Beckett, Lord Grimthorpe.

Clock wise

When we was not haranguing people in court, Beckett liked to wile away his time building clocks and interfering in public works.

The rebuilding of Westminster Palace, following a fire in 1834, gave the flamboyant barrister the chance to indulge both his hobbies.

Author of the catchily titled Rudimentary Treatise on Clock and Watchmaking, Beckett created a revolutionary "double three-legged gravity escapement" mechanism for the Parliamentary timepiece.

The design has became the model for virtually all subsequent tower clocks.

The project turned into a busman's holiday for Beckett, when he was sued for libel by the makers of the clock's bells.

The name "Big Ben" actually refers to neither the clock nor the Gothic Revival tower, but to the largest of the five bells inside - the bell which chimes the hour.

Chime time

The 13-tonne bell was named after Sir Benjamin Hall, the commissioner of works when the clock was installed.

Big Ben is famous around the world.

Its thunderous chime is heard at 1200 GMT on the BBC World Service and was a staple of TV viewing when it opened the now defunct ITN News At Ten.

Even if the clock performs perfectly on New Year's Eve, the bell may be the proverbial spanner in the works.

In 1997, sub-zero temperature coated the bell hammer with ice. A repeat of the resultant "Ding-dannggg" is unlikely to please new year's revellers.

The bell may experience an even more catastrophic failure, it could theoretically split.

The Whitechapel Foundry, which cast Big Ben, also made the famously flawed Liberty Bell.

The traditional symbol of US freedom broke while being tested in Philadelphia in 1752.

Recast, it rang for four days to celebrate the first reading of the Declaration of Independence, only to crack again during festivities to mark George Washington's birthday in 1846.

Should Big Ben suffer a similar fate before 1 January, the clock keepers may well chose to seek refuge in one of the tower's more novel features - a prison cell.