* Company calls YPF nationalization unlawful, discriminatory
* Repsol values its 51-percent stake in YPF at $10 billion
* Gov't has established its own valuation panel
By Alejandro Lifschitz
BUENOS AIRES, Dec 3 (Reuters) - Spanish oil major Repsol filed a complaint against Argentina at the World Bank's international arbitration forum on Monday over the government's expropriation this year of Repsol's controlling stake in YPF.
Argentine President Cristina Fernandez nationalized YPF , the country's biggest energy company, in May, accusing Repsol of slack investment to boost output at a time of growing demand.
Repsol, which has denied under-investing in YPF, confirmed its request for arbitration at the International Center for Settlement of Investment Disputes (ICSID) in a statement on Monday.
The oil company's filing calls Argentina's seizure of its 51 percent controlling stake in YPF "unlawful and discriminatory," a legal source said.
Repsol says the stake is worth $10 billion but Argentine government officials have indicated they think it is worth significantly less. The Planning Ministry has formed a committee to determine its value for compensation purposes.
Legal specialists say it could take more than a year to complete the ICSID arbitration process, which deepens legal wrangling over the nationalization.
Repsol also sued Argentina in May through a U.S. court and has threatened legal action against companies that invest in YPF.
A Spanish court last month agreed to consider a lawsuit by Repsol against U.S.-based Chevron Corp over its cooperation agreement with YPF.
Argentina has a poor record at the ICSID.
In March, U.S. President Barack Obama said he would suspend trade benefits for Argentina because it has failed to pay more than $300 million in compensation awards in two disputes handled by ICSID.
Fernandez's takeover of YPF drew strong criticism from the European Union and several other key trade partners already irked by Argentina's import curbs and other unorthodox policies aimed at boosting the trade surplus.
YPF, under new chief executive Miguel Galuccio, has unveiled a $37.2 billion investment plan through 2017 aimed at boosting oil and natural gas output by almost a third and bringing the company's huge shale resources on stream.