Significant pressures in some local NHS areas are being masked by national statistics but overall performance is holding up well, according to the King's Fund.
The charity's latest quarterly monitoring report on NHS performance has found a significant minority of hospitals are becoming more stretched amid funding cuts.
These trends are identified by both statistics from the Department of Health (DoH) and opinions from finance directors of NHS trusts.
According to the report, although average waiting times remain within the target range, 1 in 4 hospitals failed to meet the target threshold for treating at least 90% of patients within 18 weeks of referral in August.
One in ten providers breached the target threshold that 95% of patients should not have to wait longer than four hours in A&E - nationally, only 3% of patients waited longer than four hours in the first quarter of 2011/12.
And while levels of hospital-acquired infections have reached a new low, 45 hospital trusts reported higher levels of C. difficile infections than the same period last year.
With the NHS financial squeeze beginning to bite, the report also highlights continuing concern about whether ambitious productivity targets can be met.
Most of the 23 finance directors questioned were uncertain or concerned about whether their trust will meet its target, with the majority expecting to face equally challenging targets of 4% or more next year.
But more positively, only one of the finance directors expects their trust to be in deficit at the end of the financial year.
The charity explained that the trend followed on from its last survey, which suggested that NHS providers are facing higher productivity targets than commissioners.
Professor John Appleby, chief economist at The King's Fund, said: ‘While the NHS continues to perform well, a minority of trusts are struggling to keep waiting lists down and reduce hospital-acquired infections.
"Looking ahead, the challenge will be to maintain performance and deliver productivity improvements as finances tighten further.
"Six months into an unprecedented four-year period of financial restraint, the pressures already emerging in a small number of trusts highlight the scale of the challenge facing the NHS,' he added.
