We present the lithium measurements of a continuing programme of light element abundances in metal-poor stars. New equivalent widths of the Li i lambda 670.8 nm resonance line in 67 metal-poor stars covering the metallicity range -3.5 <= [Fe/H] <= -0.4 are reported. For about half of this sample, the observations presented here represent the first measurement of the Li i line. The sample allowed a statistical comparison with previous measurements from other authors and a study of the consistency and reliability of the quoted error bars. This paper shows that for most of the stars these error bars are good estimates of the true uncertainties associated with the determination of the equivalent widths of the Li i line. However, about 20% of the stars with two or more independent measurements show discrepancies in the Li i equivalent widths; in these cases, other sources of uncertainty not properly taken into account (binarity effects, cosmic rays, imperfect flat-field correction, continuum determination, etc.) could also be important. Conclusions on the possible lithium abundance trends versus effective temperature or metallicity and on any intrinsic scatter should be treated cautiously until their robustness vis-a-vis these additional uncertainties is proved. Based on observations made with the Isaac Newton and Nordic Optical Telescopes, which are operated on the island of La Palma by the Isaac Newton Group and the NOT Scientific Association, respectively, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.