UPDATE 1-Schiphol airport, union deal to cost 1 in 5 jobs

* To cut 450 jobs; up to 550 had been threatened

* Airport owner pledges no forced redundancies

(Adds Finance Ministry spokeswoman's comments)

AMSTERDAM, May 19 (Reuters) - One in five jobs will be lost at Schiphol after unions signed up to the Amsterdam airport operator's staff reduction plan in exchange for a guarantee of no forced redundancies.

State-owned Schiphol Group [SCHP.UL], which runs the fifth-largest passenger airport in Europe, said in January it would cut between 10 and 25 percent of its 2,200-person workforce following a strong decline in traffic and increased international competition.

It said on Tuesday it reached a deal with three unions to shed 450 jobs -- or about 20 percent of the total -- by the end of 2010. In exchange for no forced layoffs, the unions also settled for a 1.75 percent wage rise.

Some 250 jobs will be cut through outsourcing and the rest via natural attrition.

The airport's net profit fell by 41 percent in 2008 to 187 million euros ($253 million) and the company warned in February it could fall by a further 50 percent in 2009.

Schiphol's passenger numbers have stagnated since a travellers tax was introduced last July. New anti-terrorism security costs and regulatory pressure to cut airport charges have also hurt profits.

The Dutch government, which owns the group together with local authorities, agreed in principle last month to scrap the flight tax on July 1 in exchange for Schiphol looking for ways to cut its own fees as soon as passenger numbers increased.

A spokeswoman for the Dutch Finance Ministry said the agreement is expected to be formalised in a letter to parliament next week, while a Schiphol spokeswoman said the airport had reduced its fees by more than 10 percent in April and would not raise them in November.

"We hope in the coming months to see an improvement in passenger numbers," spokeswoman Mirjam Snoerwang said, adding that Schiphol had lost an estimated 1.3 million passengers as a result of the flight tax.

"We hope without a flight tax from July 1 they will start to return," she said. ($1=.7378 Euro) (Reporting by Aaron Gray-Block; editing by John Stonestreet)