Restrictions or bans on some internships are “an assault on volunteering by the back door” and should be resisted by charities, according to Joe Saxton, driver of ideas and founder of the research consultancy nfpSynergy.

In a blog post published yesterday on the nfpSynergy website, Saxton says it can be difficult to differentiate between a volunteer and an intern, and charities should therefore be concerned at attempts to outlaw some internships.

His warning comes as new polling data released by the Social Mobility Commission on Monday says 72 per cent of the public back a change in the law on internships, with 42 per cent of respondents saying they strongly support a ban on unpaid internships lasting longer than four weeks.

The poll contacted 5,000 people and was conducted by YouGov, the Social Mobility Commission said. The commission is an independent body that monitors progress in improving social mobility.

A private member’s bill proposing a ban on internships that exceed four weeks is due to be discussed in the House of Lords on Friday.

But Saxton’s blog says that restricting volunteering to the over-30s, with under-30s classed as interns, would affect people who volunteer at a young age and work abroad or because of a religious or political conviction.

Saxton writes: “Perhaps one way would be that we ban internships with companies, but charities are OK. What about volunteering vs internships with the NHS? Or an MP? Or a party political campaign? Or a campaign group? Are those OK? So it’s OK to intern for anything as long as it’s not a company?

“Of course we should encourage any organisation which can afford to pay interns to do so. But the idea that unpaid internships should be banned is an assault on volunteering by the back door. And the voluntary sector should resist it with all its might.”