Teachers will struggle to reduce their workload unless schools increase their class sizes, a new analysis has claimed.

A report by the Education Policy Institute (EPI), published today, found it was “unlikely” teachers can cut down on workload unless classes are expanded so they have more time to prepare for fewer lessons.

Staff in countries with fewer, larger classes such as China are able to prepare better for lessons – and also spend 10 times longer on career development – than their English counterparts, the data shows.

Peter Sellen, chief economist for the EPI, said: “Parents want schools with small classroom sizes, but what the evidence shows is this may mean teachers actually have less time to prepare for all of their lessons.

“We need to think again about investing capital into schools with small classroom buildings, especially while there’s not the funding to employ many more teachers.”

In a new analysis of 2013 teacher feedback from OECD countries, the report found that English teachers spend about 24 minutes preparing for one hour of lessons.

This was “significantly lower” than Shanghai, an education system regularly praised by schools minister Nick Gibb.

In Shanghai, which has larger class sizes than English schools, teachers prepare at least 35 minutes per hour of lessons.

But speaking in the summer Gibb admitted that Shanghai teachers only deliver two lessons a day whereas a teacher in England will typically teach at least four.

“They use the afternoon to help pupils on a one-to-one basis and that is a luxury that Shanghai has,” he added, before saying that England would be unable to replicate the policy.

Teachers in Shanghai also had more time to invest in their professional career development as a result of their lessened teaching hours, the report showed.

While English teachers receive on average four days CPD a year, those in Shanghai were undergoing training and feedback for an average of 40 days a year – while the OECD average was 10 days.

Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said the lack of CPD for teachers was one of the “most worrying” findings in the report.

“Teachers want to do the best they can for their pupils, but they are being held back by ‘busy work’ and a lack of training and development which would enable them to meet the challenge of change which, for many, is overwhelming.”

A Department for Education spokesperson said:

“We want every child to have access to great teachers that aren’t bogged down with unnecessary workload so they have the time and freedom to do what they do best – inspire the next generation. We recognise teachers’ concerns and are continuing to work with the sector to find constructive solutions to this issue.

“Teaching remains an attractive career and we have more teachers entering our classrooms than those choosing to leave or retire. Teacher retention has been broadly stable for 20 years and the annual average salaries for teachers in the UK are also greater than the OECD average, and higher than many of Europe’s high-performing education systems like Finland, Norway or Sweden.”