* Ban on layoffs for at least 30 months
* State to keep 34 pct stake
* Other conditions include keeping base in Portugal (Adds quotes, other conditions of sale)
By Andrei Khalip
LISBON, Jan 15 (Reuters) - The future buyer or buyers of Portugal's flag carrier TAP will be barred from laying off workers en masse as long as the state remains a shareholder, the government said on Thursday.
The Portuguese government set the condition after talks with the unions who last month threatened a crippling strike over Christmas, forcing it to invoke special labour requisition powers to prevent the stoppage.
The government plans to sell its 66 percent controlling stake in TAP, but will retain a 34 percent stake that can be sold off two years after the privatisation.
"The agreement (with the unions) guarantees that there will be no collective layoffs for a certain period of time - for 30 months or as long as the state has a stake in the company after this privatisation, whatever is the longest," said the state secretary for transport, Sergio Monteiro.
At a cabinet meeting on Thursday, the government approved a prospectus with conditions for TAP privatisation, which also include preserving TAP's status as the Portuguese flag carrier and keeping the headquarters and main hub in Lisbon.
The airline has debts of some 1 billion euros and the cash-strapped state is banned from injecting fresh capital into the company under European Union rules. Prime Minister Pedro Passos Coelho has said TAP will not survive if it remains in state hands.
The privatisation is part of a programme of state-property sell-offs agreed between Lisbon and its European and International Monetary Fund creditors under its 2011 bailout, which the country exited last year. Most other privatisations have been successfully completed, exceeding Lisbon's revenue target.
TAP's sell-off is unlikely to bring much cash to the government, but would rid it of the airline's sizeable debt.
"We are not looking to raise cash with this privatisation, but for conditions for the company to continue to grow in the future," Monteiro said.
The TAP privatisation was relaunched in November after being suspended in 2012 when the only bidder failed to present financial guarantees in time. The government hopes it can sell TAP this time after being approached by at least three potential bidders. Media reports cite Brazilian airline companies Gol and Azul among potential bidders. (Editing by Jeremy Gaunt)