A 27-year-old headteacher controversially appointed to take over a free school despite having no teaching qualifications has left her job, the Guardian can disclose.

Annaliese Briggs was appointed principal of Pimlico primary in central London in March by a charity set up by a government minister. She had no teaching qualifications and little experience in running a school. The new free school is sponsored by the Future Academies charity set up by Lord Nash, a junior schools minister and one of Michael Gove's closest allies.

On Wednesday Pimlico primary confirmed that Briggs had left her post to "pursue other opportunities in primary education". She will become a governor at the school instead. The school began admitting pupils for the first time last month.

Sources close to the academy say she was finding it difficult to cope with the workload. One local teacher, who did not want to be named, said he was surprised that such an inexperienced candidate had been selected.

"She was not happy because she could not cope with the job, full stop. It was too much to learn, too quickly."

Pimlico primary opened in September to 60 pupils, on the site of Pimlico Academy, a mile from the Houses of Parliament.

Briggs, an English literature graduate from Queen Mary, University of London, had worked as a junior member of staff at the rightwing thinktank Civitas. She had no qualifications when appointed but was reportedly trained in Wandsworth in preparation for the beginning of the school year. She said that she would ignore the national curriculum and teach lessons "inspired by the tried and tested methods of ED Hirsch Jr", the controversial American academic behind what he calls "content-rich" learning.

Kevin Brennan, the shadow schools minister, said Briggs's decision to stand down raised questions for the government. "Parents will be worried that David Cameron is presiding over a dumbing down of standards in schools by allowing unqualified teachers into our classrooms. Labour wants to see social entrepreneurs and educational innovators injecting dynamism into our schools but not at the expense of rigour and high standards."

Gove has called for more people without teaching qualifications to take over schools. In 2010, the education secretary announced that free schools – which are outside the control of local authorities but funded by the state – would be allowed greater leeway over appointments.

Last summer he extended such freedoms to the country's 1,500 academies, claiming that removal of the requirement for staff to have qualified teacher status (QTS) would replicate the "dynamism" that he believes is found in private schools. Briggs had disputed that she had no relevant experience and had been chosen for her curriculum expertise. She also said she taught primary school children in supplementary schools – which offer out-of-hours teaching – helping improve literacy and numeracy skills.

Paul Dimoldenberg, leader of the Labour group on Westminster council, said the appointment was "ideological favouritism" and Briggs should not have been put in such a difficult position.

"Being a headteacher is a tough job that can be very stressful. By giving the job to a person with no teaching qualifications and without the necessary experience, the authorities may have contributed to a difficult situation for her and the parents, staff and pupils at the academy," he said.

When the Guardian approached the academy yesterday morning, a spokesperson refused to confirm or deny Briggs' departure. Parents who left the school at 3.30pm said they had been told she was ill and expected her to return.

Nash also initially declined to say whether she had left her post, but added that his relationship with Briggs remains "perfectly friendly" and declined to comment further. But the school confirmed later that Briggs was leaving. Staff at Millbank school, which is also within the academy group, had been told about the changes on Tuesday.

A spokesperson for Future Academies said: "Having successfully set up Pimlico primary, Annaliese Briggs has decided to leave Future Academies to pursue other opportunities in primary education."

A new principal has been appointed who "is already known to the pupils and families of Pimlico primary, and brings over 10 years experience in both early years and leadership".

• The picture on this article was changed on 10 October 2013. The original picture was of the old Pimlico comprehensive school, which has been demolished. Although the original caption accompanying the picture explained that it was the old school, the image has now been removed to avoid further confusion.