The architect of the government's decision to increase NHS spending by £40bn will today cause anxiety across Whitehall by launching an independent inquiry into how well the money was spent.

Sir Derek Wanless, the former NatWest chief executive who provided Gordon Brown with the economic justification for NHS growth, has accepted an invitation from the King's Fund to investigate whether the extra money produced sufficient improvement in the nation's health.

In 2002, Sir Derek's review of the NHS for the Treasury paved the way for unprecedented levels of funding, with annual increases worth more than 7% in the health budget sustained for the five years to 2007-08. Now he will conduct a fresh six-month inquiry into the results. He intends to report in April or May - in time for his conclusions to influence the Treasury's comprehensive spending review.

His conclusions will be published at a potentially awkward time in the handover of power from Tony Blair to a successor.

Sir Derek said in the 2002 report that the government should commission an independent review of the NHS every five years. He is understood to be doing the job for the King's Fund, an independent research institute, because there was no enthusiasm in Whitehall for the project.

The fund said: "He will examine how changes in the health of the population, demographics, productivity, staff pay, drug costs and medical advances have influenced healthcare spending, as well as analysing how the NHS has spent extra resources."

Niall Dickson, the fund's chief executive, said Sir Derek would keep an open mind on NHS performance until he had assessed the evidence.