The PMI, compiled by the Chartered Institute of Purchasing and Supply, said there were signs that order book growth was nearing stagnation as activity fell to its lowest level since February 1999.
The PMI fell to 52.5 in October, down from 54.2 in September.
A level of 50 shows there is neither expansion nor contraction.
The latest index showed that, although the UK construction market has grown for the 33rd consecutive month, the rate of growth had slowed for the fifth month in a row.
Only the civil engineering sector grew at a faster rate in October compared with the previous month.