A former aide to a Liberal Democrat peer has been condemned for charging former unpaid interns at his thinktank “£300 a go” for employment references.

Jan Mortier, who describes himself as a former consultant to Lord Garden, a one-time defence spokesman for Nick Clegg’s party, has admitted that he charges former unpaid trainees at his Civitatis International organisation for references, but denied that they had been interns, on the basis that they had been “trained directly” by him.

Civitatis International advertises itself as a private foundation “committed to promoting peace, dialogue and co-operation between nations and civilisations”, and has submitted evidence to parliamentary select committees as a thinktank. Until a year ago it ran a “junior associates” programme under which young people were charged over £1,600 for a three-month “unique experience in project management training at our international secretariat in the City of London that was instituted by us because British universities are not giving the skills or experience necessary to help young people secure careers in the policy sector”.

The junior associates programme, which did not offer a recognised qualification at the end or a guaranteed job, had been advertised on a website called Internwise, among others, which promotes itself as a “tool ideal to meet employers and gain some work experience”. At least one former junior associate has posted an online CV describing his role at Civitatis International as an “internship”. Civitatis invites “successful” junior associates to pay an additional £400 to £600 a year to become fellows of the organisation, which it describes as a private members’ club for “future leaders”.

Now it has emerged that Mortier, 37, has written to those who had been on the junior associate programme to inform them they must pay a £300 fee each time they want an employment reference.

Tanya de Grunwald, the founder of Graduate Fog, a graduate careers blog and a campaigner against the exploitation of the young, last night condemned Mortier and his organisation and said it was an extreme example of how the hopes of young people were abused. There has been a huge growth in unpaid internships in recent times, with an estimated 100,000 places advertised a year.

De Grunwald said: “Employing unpaid interns is bad enough, but charging them for a reference when they leave is appalling. We keep being assured that the graduate job market is picking up, but this case shows that there are still dark corners of it where unscrupulous employers find they can take advantage of young jobseekers’ desperation and naivety. This guy should be ashamed.”

Civitatis’s website says it was founded in 2012. A company with the same name, of which Mortier was a director, was struck off Companies House records in 2009 after being dissolved. It is not registered with the Charities Commission. Mortier declined to comment on the organisation’s tax status.

Civitatis’s website is advertising a summer school at a cost of £400 for the week. Those who attend are promised a “once-in-a-lifetime opportunity for students around the world to gain employable skills”. It claims that it can offer “an introduction to the thinking of the Club of Rome”, the global thinktank where Mortier claims on his Linkedin profile to have “advised the secretary general on various issues”. A spokesman for the Club of Rome told the Observer: “Jan Mortier was an intern at the Club of Rome for five months in 2010. He left a month early following a dispute. There is no link between the Club of Rome and Civitatis International.” A spokesman for Civitatis said Mortier was a “full member of the Club of Rome EU chapter”, an affiliated Belgian organisation.

Civitatis’s website claims that “for a decade, Civitatis International has been coaching our junior associates to get policy jobs paying £24-£32,000 per year with a 100% success rate”. When approached by the Observer, Mortier admitted that “one or two” alumni might not have reached their goals yet. He said the £300 fee for an employment reference was a “fair administrative fee”.