Germany's FDP agrees tax-cutting election platform

* FDP leader makes joining coalition conditional on tax cuts

* FDP hopes manifesto will spring it to power

HANOVER, Germany, May 17 (Reuters) - The leader of Germany's opposition liberal Free Democrats (FDP) on Sunday made tax cuts a condition for his party joining a coalition government after September's election.

At a party congress in Hanover, the FDP agreed a campaign manifesto centred around a tax cut pledge that it hopes will spring the party, Germany's main opposition, into power.

The pro-business FDP has been the biggest political winner from the financial crisis. Polls show it could win enough votes at the election to take power with Chancellor Angela Merkel's conservatives, who share power with the Social Democrats (SPD).

"We liberals will only sign a coalition agreement if it stipulates a low, fair and simple tax system," FDP leader Guido Westerwelle said at the end of the party congress, adding that the conservatives were the FDP's preferred coalition partner.

An ARD poll this week put the FDP down one point at 13 percent, the conservatives at 35 percent and the SPD 28 percent.

The FDP centred its programme around a pledge to simplify the tax system and reduce the burden on families and middle income earners by up to 35 billion euros ($47.40 billion).

The party has shared power with both conservatives and the SPD since 1945. It is now in coalitions with conservatives in five German states, with about two-thirds of the country's population, and the two parties broadly agree on many financial and energy policies.

Merkel has made tax cuts a central part of her campaign for the federal election in September, although she has played down the prospect of tax relief before 2012 under pressure from members of her own party.

Germany is facing its deepest recession since World War Two this year and the slump is hitting public finances.

The combination of declining revenues and higher spending on jobless benefits and stimulus measures is expected to rip a huge hole in the German budget, pushing the deficit above EU limits to about 3.9 percent of gross domestic product (GDP) this year. (For a FACTBOX on the FDP's election programme please double click on [ID:nLG262214]) (Reporting by Sabine Siebold; Writing by Paul Carrel; Editing by Louise Ireland)