The proportion of patients waiting more than 18 weeks for treatment has increased, official figures show.

The terms of the NHS Constitution state that patients should be seen within 18 weeks.

Data for May 2011 reveals that 2.3% of outpatients waited more than 18 weeks compared to 1.8% in May 2010. And 9.2% of inpatients waited more than 18 weeks compared to 7.1% last year.

NHS chiefs insist "the percentages fluctuate but remain low" and they are committed to keeping waiting times low "despite rising NHS activity".

The government said there were over 100,000 more diagnostic tests in the three months to May 2011 than in the three months to May 2010.

According to the figures, the vast majority of patients were referred to treatment within 18 weeks: 90.8% of hospital inpatients and 97.7% of outpatients started treatment in 18 weeks or under from referral, the Department of Health said.

Figures reveal the average waiting times for admitted patients being treated was 8.4 weeks in May 2010 and remains at 8.4 weeks in May this year.

Health Secretary Andrew Lansley said: "We are determined to keep waiting times low.

"The first full year of NHS waiting time data under this Government confirms that the proportion of patients who wait longer than 18 weeks for treatment remains low and stable.

"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."