The Conservatives said the figures were "very disappointing"
The number of outpatients in Wales waiting for first hospital appointments has risen sharply according to the latest official figures.
Those waiting more than 10 weeks for a first outpatient appointment rose by 2,668 to 5,840 last month.
Opposition politicians are demanding explanations for the "spike" but the assembly government said the figures did not show bigger picture.
One health economist warns it could be tougher to hit future targets.
Waiting times have been reducing recently as ministers try to reach the levels in England.
But the numbers waiting more than 14 weeks for hospital admission for day care treatment or as an inpatient also rose in July by 432 to 3658.
Welsh health minister Edwina Hart AM has set a total target time of 26 weeks for people to wait for treatment by the end of December.
Now there is a concern the sharp rise in July's figures may make it tougher to reach that target.
"You are going to get people backed up waiting for treatment to begin," said health economist Siobhan Mclelland.
"This rise could be because health managers are concentrating on the huge re-organisation of local health boards, or because of financial pressures," she said.
OUTPATIENT WAITING TIMES
Patients waiting more than 10 weeks in June - 3,172
Patients waiting more than 10 weeks in July - 5,840
For first appointments in Wales. Source: Statistics for Wales
A Welsh Assembly Government spokesperson said: "These waiting times refer only to specific components of waiting times and do not give the bigger picture.
"The assembly government has invested in more staff, new buildings and equipment, and the NHS is delivering for patients"
But Welsh Conservative health spokesman Andrew RT Davies said the figures were very disappointing because figures had risen across the board.
"This surge is unacceptable - especially during the summer months, which are traditionally quieter.
"The minister needs to explain why with the waiting time figures dropping so low earlier this year now, just months later, the numbers waiting are spiralling," he said.
Welsh Liberal Democrat health spokesman Peter Black AM described the figures as shocking.
"I do not understand why there can be such drastic increases in the number of people waiting for treatment from one month to the next," he said.
"So much progress was made at the end of the financial year but it is now in danger of being completely wiped out."