Thousands of independent school teachers could take to the picket line over changes to their pensions, in what would be the first ever national strike to hit the sector.
More than 25,000 ATL and NUT union members who teach in private schools are being balloted on whether to take "unprecedented" industrial action.
Teachers and union officials told The TES they are "very angry" over proposed alterations to the teachers' pension scheme, which would see their contributions rise by 50 per cent. They said they expected members to back the strike.
Mass strikes would cause chaos for heads - more than two-thirds of teachers at prestigious schools such as Dulwich College and Berkhamsted School are ATL members - and could force some schools to close.
Independent school teacher Geoff Pye, a member of the ATL's national executive, said he expected members to join their state counterparts, who are being balloted separately, on the first national strike in the moderate union's 127-year history.
"We are forced into a position, I think, where independent school teachers are very reluctantly thinking we have to go along with industrial action," he said.
Neil Croally, who represents more than 100 ATL members at Dulwich College in south London, said he expected them to vote in favour of the strike.
"It is a concern for the sector as a whole. We would have some real difficulties if these people went on strike. It would certainly be difficult to manage, from the (school) management's point of view," he added.
David Wiles, ATL representative at Berkhamsted School in Hertfordshire, said the "catastrophic" impact of the proposed changes had created "unanimity of hostility towards the Government".
"Independent school teachers are just not militant," he said. "The fact they are even thinking about striking is unheard of."
Helen Porter, who teaches at an independent school in the South West, said teachers in the sector "feel they have most to lose", with Lord Hutton's review of public sector pensions suggesting that only state school teachers should be eligible for the pension scheme.
John Richardson, ATL's national official for independent schools, said this would create a "Berlin Wall" blocking movement between state and private schools, with a "massively detrimental effect on the independent sector".
"Members are very angry. We know it would be absolutely unprecedented if they vote in the independent sector for a strike," he added.
Under the Government's proposals, contributions would rise from 6.4 per cent of salary to an average of between 9.5 and 9.8 per cent by 201415. A teacher earning pound;35,000 a year would have to pay an extra pound;100 a month. Pensions would also switch from the final salary scheme to career average, with the retirement age being gradually raised.
The ATL and NUT ballots close on 14 June. Union leaders will then decide whether to proceed with the first day of strikes on 30 June.
The NASUWT is awaiting further talks with the Government before balloting members, while the NAHT and ASCL heads' unions and Unison, which represents support staff, have also threatened to take action.
Girls' Schools Association president Helen Wright said strikes in independent schools were "certainly a possibility".
Dr Wright, head of St Mary's School in Calne, Wiltshire, added: "It has united the teachers across the state and independent sectors. The boundaries are blurred in a way they have never been before."
Bernard Trafford, head of the independent Royal Grammar School in Newcastle, said it would be "somewhat perverse" for private school teachers to take part.
"We're not interested in upsetting our parents who pay fees. It would seem rather pointless for independent school teachers to strike when it wouldn't affect the Government," he said.
UNITY IS STRENGTH: DAY OF RECKONING
On Thursday 30 June, 500,000 public servants could go on strike.
The 250,000-strong Public and Commercial Services Union is to ballot its members on strike action.
The ATL and NUT are taking the same line, and the NASUWT and the University and College Union may jump on board if TUC negotiations with the Government fail to improve the pensions offer for public sector workers.
`CONCERNED': REFORM WARNING
The Commons spending watchdog has raised "concerns" over the Coalition's reforms to public sector pensions.
In a report issued yesterday, the Public Accounts Committee warned the Treasury about not considering the impact of the changes, particularly on attracting people to jobs such as teaching. Chair Margaret Hodge said: "We are concerned that the Treasury has not set out clearly what level of spending it considers sustainable in the long term. Instead, officials appeared to define affordability on the basis of public perception."
The NUT said the report "shattered the Government's case for further cuts in public sector pensions".
Original headline: `Unprecedented': independents could join strikes on pensions