Unions plan to stage 19 days of strikes in Spanish airports between April and July in protest against government measures to privatise national airport operator AENA, a union spokesman said on Tuesday.

Spain said in December it wanted to partially privatise AENA, which it says could be worth up to 30 billion euros ($42 billion), alongside its state lottery, as part of plans to to reduce the national debt.

"Initially, we're talking about 19 days between April and July," said a spokesman for USO, one of the unions which represents AENA employees.

Wildcat air traffic controller strikes in early December stranded thousands of passengers causing chaos at Spanish airports and prompting the government to bring in the army to take over air control.

As part of the privatisation plan, the government would allow private companies to take stakes of up to 49 per cent in the country's airports and airport services' business, a sale that is likely to draw both national and foreign interest.

Spain's biggest airports, Madrid and Barcelona, would be run privately under operating concessions, Prime Minister Jose Luis Zapatero said at the time.

The privatisation could be completed before the end of the first half of 2012. – Reuters