London City Airport has been declared safe after emergency services found no evidence of a suspected chemical incident.

"Two complete sweeps of the airport building were carried out jointly by firefighters and police officers both wearing protective equipment," the Fire Brigade said.

"No elevated readings were found and the building was ventilated, searched and declared safe," it added.

Earlier, hundreds of passengers and staff were evacuated from London City Airport following a "chemical incident" that reportedly left 26 travellers feeling unwell and shut down flights in and out of the hub.

"We are treating 26 patients at the scene for difficulty breathing," the London Ambulance Service said, adding of those two had been taken to hospital.

A "hazardous area response team" was sent to the airport in east London, which caters mainly for business travellers and short-haul flights.

"Around 500 members of the public and staff have been evacuated and there are reports of a number of passengers feeling unwell," the Fire Brigade said in an earlier statement.

London City Airport's official Twitter account said it had been evacuated "due to fire alarm".

Several incoming flights from destinations such as Frankfurt, Amsterdam and Paris were diverted to other airports.

London City Airport is primarily aimed at business travellers and is the 13th busiest airport in Britain, with a total of 4.3 million passengers last year.

Elsewhere in London, a 19-year-old man was arrested by counter-terrorism officers after a device was discovered on a Tube train yesterday sparking a major security alert.

The suspect was seized in the street this afternoon in an armed operation on Holloway Road, north London.

A Taser was discharged during the arrest but no shots were fired.

The swoop came after a controlled detonation was carried out on the suspicious item at North Greenwich station, close to the O2 complex in south-east London yesterday morning.

In a statement, Scotland Yard said: "The 19-year-old man was arrested on suspicion of the commission, preparation and instigation of terrorism acts under Section 41 of the Terrorism Act 2000.

"He has been taken to a London police station where he remains in custody."