WESTMINSTER, LONDON—Causing chaos along the longest river in England, the UK government declared a national emergency Monday after a singular Weetabix biscuit was thrown into the River Thames, instantly soaking up the entirety of the waterway.

"I want to reassure the public that the situation is under control," said British Prime Minister Andy Burnham, following an emergency COBRA meeting with executives from Weetabix parent company Post Holdings. "We are working at pace to refill the River Thames as we speak. The process will take some time, as removing the soggy Weetabix has proven exceptionally difficult. I have deployed the Royal Marines and SAS to help aid in the extraction and cleanup efforts, while GCHQ and MI5 are leading the hunt for the perpetrator, who remains at large. We are asking the public to report anyone they see carrying boxes of Weetabix to the authorities immediately."

According to Scotland Yard, Met Police officers stationed at Westminster Bridge witnessed a man in his late 40s wearing a black hoodie and joggers hurl a single Weetabix biscuit over the side of the bridge. The officers reported that the breakfast cereal began to absorb the entirety of the waterway within mere seconds, leaving behind an empty chasm with mountains of sewage dumped into the river by Thames Water.

"Whoever decided to do such a callous act in London will face the full force of the law," said London Mayor Sadiq Khan following the COBRA meeting. "The Underground and bus services are closed to the public until further notice to prevent the criminal from escaping the capital. The Metropolitan Police and King's Guard have assured me they are chasing every lead and sweeping every corners of our city to ensure this hooligan is brought to justice. Mark my words: we most certain have had our Weetabix."

At press time, Police Scotland reported that a man fitting the suspects description was spotted throwing a second Weetabix biscuit into Loch Ness, draining the lake in mere seconds, forcing Nessie to pack her bags and find another loch to live in.