FACTBOX - The NUT national strike

LONDON (Reuters) - Thousands of schools were forced to close on Thursday as teachers staged their first national strike for two decades.

Here are some facts about the strike:

WHO IS STRIKING?

The one-day strike, the first for 21 years, involves teachers from the National Union of Teachers (NUT), which has about 200,000 members. They voted by 3 to one in favour of the industrial action. However no other teaching union is striking.

The Local Government Association (LGA) estimates that, as there were about 432,000 full time regular teachers in January 2007, about 11 percent of the regular school workforce had voted for action.

WHAT IS THE STRIKE ABOUT?

The government announced in January it would increase teachers' pay by 2.45 percent in September, with further 2.3 percent rises in September 2009 and 2010. The three-year deal was recommended by the independent School Teachers Review Body.

While other unions such as the NASUWT and the Association of Teachers and Lecturers said they accepted the recommendation, the NUT said the rise was below the Retail Price Index inflation rate, and so amounted to a pay cut.

It argues that the pay award sapped morale, youngest teachers would be hardest hit, and it would deter graduates from applying to join the profession.