By Claudia Rebaza and Catherine E. Shoichet CNN

(CNN) -- London City Airport was evacuated on Friday after a number of passengers reported feeling unwell, police said.

Reports of a chemical incident sent three fire engines and "a range of specialist appliances" to the scene, the London Fire Brigade said in a statement.

Officials said the cause of the incident is unknown.

"The airport has been evacuated as precaution," London's Metropolitan Police said.

The city's ambulance service said it was treating 26 patients at the scene for difficulty breathing. Two patients were taken to the hospital, the service said.

Developing story - more to come

TM & © 2016 Cable News Network, Inc., a Time Warner Company. All rights reserved.