MONTREAL (REUTERS) - The UN aviation agency has prohibited shipments of lithium-ion batteries as cargo on passenger aircraft, following concerns by pilots and plane makers that they are a fire risk.

Lithium-metal batteries, which are used in watches, have already been banned on passenger planes globally. They are not rechargable, while lithium-ion batteries, used in cell phones and laptops, can be recharged.

The International Civil Aviation Organisation's 36-state governing council said the prohibition would be in effect as of April 1, and would be maintained until a new fire-resistant packaging standard is designed to transport the batteries.

Lithium-ion batteries can still be transported on cargo planes.

The new packaging standard is expected by 2018, ICAO Council President Olumuyiwa Benard Aliu said in a statement.

The ban would be mandatory for ICAO member states.

Pilots and aircraft manufacturers are concerned that existing standards are not strong enough to contain lithium battery fires.

A 2015 working paper by an organisation representing plane makers like Boeing Co found current firefighting systems on airliners could not "suppress or extinguish a fire involving significant quantities of lithium batteries".

But one dangerous goods expert familiar with ICAO's thinking questioned whether a ban on lithium-ion batteries would really make passenger planes safer. He said instances of such battery fires usually involved deliberate mislabeling by shippers.

"When the industry banned the shipment of lithium-metal batteries, we saw instances of them being passed off as lithium-ion batteries," said the expert, who was not authorised to speak publicly.

"Those people who are not complying now won't comply with a prohibition."