By Pairat Temphairojana
BANGKOK, Feb 21 (Reuters) - Thousands of Thai farmers threatened to head for Bangkok's main airport in their tractors on Friday in protest against non-payment in a controversial rice subsidy scheme, heaping pressure on Prime Minister Yingluck Shinawatra to step down.
The rice programme was among the populist policies pioneered by Yingluck's billionaire brother, Thaksin Shinawatra, a former prime minister central to a conflict that has divided Thais for years and triggered protests, violent at times, that have paralysed parts of the capital for weeks.
It was not immediately clear what the farmers planned to do at the airport or how long they would stay, but the convoy evoked memories of 2008 when anti-Thaksin protesters blockaded Bangkok's airports and held crippling rallies against two Thaksin-backed governments.
"We are not sure where we will set up camp, but we will not leave the capital until we are paid for every grain of rice sold," former member of parliament Chada Thaiseth said on Thursday.
Media said the farmers from the central plains expected to negotiate with Yingluck and would give her until midday (0500 GMT) before heading for the huge, hi-tech airport built on a snake-infested swamp north of Bangkok.
The rice programme is critical to Yingluck's support base in the poorer north and northeast.
Generous subsidies for farmers were a centrepiece of the platform that swept her to power in 2011, but have left Thailand with vast stockpiles of rice and a bill it is struggling to fund.
Opposition leaders say the scheme is riven with corruption and losses to the taxpayer, estimated at 200 billion baht ($6 billion) a year, have fuelled protests against her government.
Yingluck and her government are being investigated by an anti-corruption panel for alleged irregularities in the subsidy scheme.
Four protesters and a police officer were killed on Tuesday when police attempted to reclaim protest sites near government buildings that have been occupied for weeks.
The protesters are seeking to unseat Yingluck and stamp out what they see as the malign influence of Thaksin, regarded by many as the real power behind the government.
On Thursday, they targeted businesses linked to the Shinawatra family, sending some stock prices lower.
"If you love your country, stop using Shinawatra products and do everything you can so that their business fails," protest leader Suthep Thaugsuban told supporters on Wednesday.
The protests are the latest instalment of an eight-year political battle broadly pitting the Bangkok middle class and royalist establishment against the mostly rural supporters of Yingluck and Thaksin.
Demonstrators accuse Thaksin of nepotism and corruption and say that, prior to being toppled by the army in a 2006 coup, he used taxpayers' money for populist subsidies such as the rice scheme and easy loans that bought him the loyalty of millions. (Writing by Nick Macfie; Edting by Paul Tait)