The industrial action is likely to affect the library's Document Supply Centre at Boston Spa in Yorkshire.

Members of the Public And Commercial Services Union (PCS) took similar action in pursuit of a pay claim on 29 July but the impact was said to be minimal.

"We will strive to make sure this is the case again," said Ian Henderson, head of document supply at Boston Spa.

"We apologise to users of the library's services for any disruption that may occur during the strikes."

PCS members accepted a pay rise of 3.6% in August 2001 on the basis that a further increase in salary would follow.

However, the union claims the current offer of 0.4% - giving an overall pay increase of 4% - is unacceptable to its members.

The last strike closed Library reading rooms

Officials at the library said they regretted the industrial action and added there were no more funds to pay workers any more than the average 4% pay rise.

Two other unions who represent staff at the library had accepted this pay offer, they added.

The library's lowest paid workers earn as little as £12,000, but bosses have insisted that the 4% rise is just the average across the staff, and those on lower incomes will receive a higher, 5.6% pay rise.

The last strike forced the closure of reading rooms in the St Pancras Building in Central London, but the library remained open.

The Newspaper Library at Colindate in North London was also open, and the document supply service at Boston Spa was unaffected.