An investigation was under way last night to discover how two British Airways passenger planes on domestic flights over the UK came close to colliding.

A Bristol to Newcastle flight came within about 1,000ft of an aircraft from Glasgow as they flew over mid-Wales at about 7pm on Wednesday.

Emergency anti-collision systems were activated and both planes were steered apart to avert the potential tragedy.

The Embraer 145 jet BA4029 carrying 43 passengers from Glasgow had been given permission to descend and land at Bristol. It had been flying at high altitude and was cleared to drop to 19,000ft.

"For reasons to be investigated, the Embraer 145 aircraft descended to below this cleared level," a BA spokesman said last night.

Meanwhile, Dash 8 aircraft BA4058, carrying 22 passengers, was leaving Bristol for Newcastle and travelling at 18,000 ft. The Traffic Collision Avoidance System (TCAS), an electronic warning device in the cockpit, was activated and advised both pilots of the actions they needed to take.

The Embraer climbed and the Dash descended.

"The TCAS system did exactly what it is designed to do so any risk of collision was eliminated," the spokesman added.

"An air proximity report has been filed following the incident involving two BA CitiExpress aircraft."

He said it was "unlikely" the passengers were aware of the incident.

An investigation is being carried out by the Civil Aviation Authority.