Less than two weeks after taking up what had been described as "the second most important job" in NHS management, Barbara Stocking has quit to become director of aid charity Oxfam. The timing of the announcement that she is to leave her post as director of the NHS modernisation agency - the body tasked with rolling out the NHS plan reforms - has stunned the health service.

There will also be disappointment that one of the NHS's most experienced, and high-profile, women managers is leaving the service, reinforcing concerns that it cannot hold on to its top female talent and role models.

It emerged this week that NHS chief executive Nigel Crisp knew that 49-year-old Ms Stocking's "hat was in the ring" for the Oxfam job - and that she would take it if offered - when he appointed her in November. Although the modernisation agency job was billed as "for an initial period of six months," and as a "secondment" from her principal job as director of NHS south-east region, there was no suggestion that she had not joined the agency for the long haul. The Department of Health website refers to her as "the new head of the agency".

But Ms Stocking, who had unsuccessfully applied for the NHS chief executive post, which went to Mr Crisp in October, told the Guardian that when he approached her about the job, she had warned him that she was considering "doing something different".

She said: "He was really anxious to get the modernisation agency moving quickly. That was why we agreed it was a secondment and that, in due course, the job would be advertised."

In what now looks ironic, Mr Crisp came under fire at the time for appearing to breach equal opportu nities policy by appointing Ms Stocking without first advertising the job.

A Department of Health spokesman said that her appointment was always intended as a stop gap: "Clearly, we felt that Barbara was an outstanding candidate to get the agency off to a flying start. It was always on the understanding that it was for a start-up period."

This still leaves the questions of why such an important position was not advertised straight away, and whether, if Ms Stocking had decided against the Oxfam job, she would have had to "re-apply" for the modernisation job next spring.

The modernisation agency was set up by Alan Milburn, health secretary, to spread good management practice across the health service and help health professionals implement the NHS plan. It has already sent teams into hospital trusts with the longest out-patient waiting lists, but will equally be working to identify the best performing organisations and reward them with more power to take decisions locally.

At Oxfam, Ms Stocking will manage a £90m budget and a worldwide staff of 2,600. She replaces David Bryer, who is leaving the charity's top job after 10 years. The post was advertised at a salary of £75,000 - some £5,000 less than she was reportedly earning four years ago when director of the NHS Anglia and Oxford region.

Ms Stocking, who lives in Oxford, where Oxfam is based, said: "I was very tempted by the modernisation job, but at the time it came up I had almost made up my mind [that she would leave the NHS]."

The lure of the Oxfam job was too great, she said. "I've always admired it as an influential and humanitarian aid agency that really works for people on the ground, in terms of their own development."