WARSAW--A key guage of Poland's manufacturing activity signaled another contraction in August, with the decline of the purchasing managers' index now the longest since that registered between March and December 2008, HSBC said Monday.

The survey of 300 industrial companies, prepared by Markit Economics for HSBC, showed the manufacturing PMI dropped to 49.0 points in August from 49.4 points in July.

A PMI figure below 50 indicates contraction, while one above that level signals expansion.

Poland's economy has stopped accelerating in recent months due to the weakness of the manufacturing sector, after a strong start to the year. The economy grew 0.6% in the second quarter from the first compared with a growth rate of 1.1% in the first quarter.

Poland's central bankers "will most likely debate policy easing in September and while it is a close call a rate cut before the end of the year is very likely," HSBC said, commenting on the weak August PMI.

Poland's benchmark rate is at 2.5% and the central bank has recently left the door open to policy easing because of a series of weaker-than-expected data.

Write to Marcin Sobczyk at marcin.sobczyk@wsj.com