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Markit Economics' final services purchasing manager's index (PMI) for May was 51.3, just under the forecast for 51.4. With the index still above 50, it indicates expansion.
However, the pace of growth was one of the slowest since the recession. Respondents to Markit's survey said client spending was weak.
They also reported weak hiring, as payroll numbers growth was the smallest since January 2015.
Markit chief economist Chris Williamson said in the release: "With optimism about the business outlook dropping to a new post-crisis low, companies are expecting conditions to remain challenging in coming months, citing uncertainty about the presidential election as well as broader worries about weak demand at home and abroad."
At 10 a.m., ISM's non-manufacturing report is due. The index is expected at 55.3.