By Costas Pitas LONDON, Jan 27 (Reuters) - Britain's biggest union, Unite, said on Monday it is to ballot 5,000 workers for strike action at car manufacturer Ford in a row over job security and pensions. Workers, mainly on the production line at Ford's six sites across Britain, will be asked whether they want to down tools in a bid to seek guarantees from the firm that there will be no more plant closures and that jobs in Britain will be protected. Last year, Ford closed its British van factory in Southampton, southern England, and shuttered an associated stamping facility in Dagenham, east London, ending its vehicle manufacturing in Britain. The firm still employs nearly 15,000 people around the country, who mainly build and develop engines, and supports a further 100,000 jobs through its network of suppliers and dealers, the company said on its website. Unite national officer Roger Maddison said staff in Britain had felt the brunt of previous closures and now wanted safeguards. "After successive rounds of job cuts including the closure of the Southampton plant, staff now want Ford to make some commitments to job security going forward," he said. The union said it also wanted improvements to the pension scheme which it said uses the consumer price index (CPI) rather than the generally higher retail price index (RPI) for inflation-linked increases to pension payments. Roughly 300 staff represented by the GMB Union will also be balloted with the vote due to close on Feb. 7, a spokesman said. In a statement, a Ford spokesman said it had a long-established practice of negotiating with unions in Britain and that further discussions with them were needed. "In 2011, a two-year agreement was reached and we re-entered discussions in 2013, recognising the priority for industry-leading levels of cost, efficiency and competitiveness," a spokesman said. "This process has been completed for some employees but is still under way between the company and unions for other areas of the workforce." Unite said it would not comment on any possible strike dates or how long action could last when asked by Reuters.