QANTAS and Jetstar planes are flying more slowly to save fuel in a desperate attempt to reduce the soaring jet fuel costs threatening airline profits.

Virgin Blue is also considering slowing down its planes during descent as a way of cutting fuel use. Jetstar has been flying more slowly since last month, adding six minutes to the length of flights from the east coast to Perth and two minutes on to flights from Sydney to either Brisbane or Melbourne, The Australian reports.

A spokesman for Qantas, which owns Jetstar, last night confirmed that its own planes had also been using "variable speed" as a "fuel conservation initiative" and said the airline had been doing this for the past two years.

The unannounced moves, which follow similar actions by US airlines, are expected to save the carriers millions of dollars and reduce upward pressure on airfares.

The revelation comes as the oil price reached a record $US133 ($138) a barrel yesterday, placing further pressure on airline fuel costs and profits.

The spike in oil has also driven petrol prices past $1.60 a litre this week, with forecasts of $1.70 next month, fuelling the political debate over the Coalition's promise to cut the petrol excise by 5c a litre. Economists, however, believe crude could soon fall back to $US100, taking pressure off petrol and aviation fuel prices.

The rising cost of jet fuel has once again prompted Qantas to increase the price of its airfares.

Qantas said yesterday its international airfares would rise by about 4 per cent and domestic fares by about 3 per cent for tickets issued in Australia from June 4. The increases follow hikes of about 3per cent for international fares and 3.5per cent for domestic fares earlier this month.

And in the US, cost pressures have become so extreme that American Airlines will now charge $US15 for checked baggage to help offset rising fuel prices.

Jetstar spokesman Simon Westaway yesterday confirmed to The Australian that a trial scheme to fly more slowly had been under way since last month as a means of combating record fuel prices, which have soared by 68 per cent in the past 12 months.

"The challenge for airlines is that fuel costs are at record levels," he said. "We have been conducting a trial where aircraft flights are taking a bit longer and burning less fuel. Early indications are that we are seeing positive savings in terms of fuel."

The move has involved Jetstar pilots reducing the average cruise speed of Airbus A320s from 863km/h to 843km/h.

These new speeds apply to all domestic Jetstar flights as well as Jetstar flights to New Zealand and may soon be extended to long-haul international flights.

Virgin Blue spokeswoman Heather Jeffrey said the company was actively exploring ways to reduce fuel use and had recently fitted winglets to all aircraft to reduce aerodynamic drag and therefore fuel use. Virgin had not slowed down its cruising speeds but was examining slowing down its speed of descent to save fuel.

A Qantas spokesman said the airline had saved fuel by using "variable speeds", which he said meant a marginal slowdown in speed.

Read the full story in The Australian