There are fears Heathrow airport could "grind to a halt" when immigration officers go on strike next week over pensions, sources have told the BBC.

Heathrow operator BAA has been holding talks with airlines and the UK Border Agency to try to minimise the impact of Wednesday's public sector strike.

A source close to the talks said more government action was needed to prevent huge queues at immigration.

The Home Office said it was exploring all options to minimise disruption.

Meanwhile, the Daily Telegraph quoted a Heathrow airport official as saying travellers arriving at the London airport could be held on aircraft for up to 12 hours while immigration queues cleared.

On Wednesday it emerged that civil servants are being asked to act as airport border staff during the strike action, which more than two million workers may join.

Strikes are being held over changes to public sector pensions and thousands of border agency workers are expected to be among those walking out.

BBC transport correspondent Richard Lister said aviation officials were working on the basis that immigration staffing levels would be at a maximum of 30-50%, and that most of those checking passports would have no experience and little training.

During the strike in the summer, border agency managers ran passport control but this time they are expected to strike too, our correspondent said.

The Home Office is pulling in border agency staff who are not union members and civil servants from Whitehall to carry out immigration checks.

In a statement, the department said it was too early to speculate on how many people would join the strike, and that it was exploring all options to minimise disruption.

"Mass Cancellations"

Normand Boivin, Heathrow's operating officer, told the Daily Telegraph that delays at immigration could be so long passengers would need to be held on arriving aircraft for up to 12 hours.

In response, carriers have been told to fly planes only half full, the newspaper reports.

"Modelling of the impacts of strike action on passenger flows at Heathrow show that there are likely to be very long delays of up to 12 hours to arriving passengers," said Mr Boivin.

"The delays at immigration are likely to be so long that passengers could not be safely accommodated within the terminals and would need to be held on arriving aircraft.

"This in turn would quickly create gridlock at the airport with no available aircraft parking stands, mass cancellations or departing aircraft and diversions outside the UK for arriving aircraft."

Disruption at the UK's largest airport is expected to be particularly severe because nearly 100 long-haul services are due to arrive before 9:00 a.m.GMT on the day of the strike.

"Fantasy Economics"

On Thursday, ministers said the strikes could cost the 500 million pounds and lead to job losses.

"That is a loss we can ill afford at what is a very very difficult time for our economy overall," said Chief Secretary to the Treasury Danny Alexander, sparking claims of scaremongering and "fantasy economics" from the unions.

Meanwhile, former Labour Home Secretary Alan Johnson has indicated he is backing next week's strike.

He is the most senior Labour figure to support the action, a stance that is at odds with his party.

"If [the unions] can't [strike] over an issue as important as their pensions, then what can they take industrial action over?" he said on the BBC's This Week.

Unions say proposals which require their members to work longer before collecting their pension and contribute more are unfair, but the government says change is needed to keep down the cost to the taxpayer, because people are living longer.