More patients are waiting over 18 weeks to be treated, according to Department of Health figures.
In May this year, 2.3% of outpatients had to wait more than 18 weeks - the limit set down in the NHS Constitution - compared to 1.8% in the same month of last year.
The figures also show that 9.2% of inpatients had to wait for more than 18 weeks, compared to 7.1% last year.
NHS bosses said that although the figures have increased, they remain low, and they insisted they are working to keep waiting times as short as possible.
They pointed out that activity in the NHS is increasing, with 100,000 more diagnostic tests carried out in the three months to May 2011 than the same quarter of 2010.
And the Department of Health said that despite the waiting time increases, the majority of patients - 97.7% of outpatients and 90.8% of inpatients - were treated within the 18-week limit.
Health secretary Andrew Lansley said: “”Average waiting times from referral to treatment are at the same levels as last year.
“This is against a backdrop of rising demand for NHS services, so the low waiting times are a testimony to the hard work of NHS staff.”
Patients Association trustee Roswyn Hakesley-Brown said: “We are deeply concerned that the efficiency savings being demanded of the NHS are having an adverse effect on services for patients, reducing services and driving up waiting times.
“We urge the Department of Health to ensure that services are properly resourced so they can provide the care that patients need.”