A Liberal Democrat MP is to be investigated by HM Revenue and Customs after advertising nine unpaid internship vacancies in the last three years, despite his party leader's call for a crackdown on the practice.

John Leech, the MP for Manchester Withington, has become the subject of a complaint after posting his latest advert last month seeking candidates with "computing and writing skills and an understanding of confidentiality". The internship, according to the ad, could be expected to last between 60 and 100 days, over three to 12 months.

Under employment law, people who work set hours, do set tasks and contribute value to an organisation are liable to be regarded by the courts as workers and entitled to the minimum wage. A spokesman for the Revenue and Customs said the department could not comment on individual cases but "all complaints of non-payment or under-payment of the national minimum wage are looked into". The complaint to HMRC was made by Tanya de Grunwald, founder of campaigning website Graduate Fog. HMRC has announced its intention to crackdown on those who use interns to carry out what should be paid work. There is growing concern that the young and unemployed are being exploited while those unable to work for free are being blocked out of valuable opportunities.

Three years ago Lib Dem leader Nick Clegg announced a ban on the use of unpaid internships, but was only able to enforce the ban in his office and party headquarters. Leech was one of the Lib Dem MPs who resisted Clegg's diktat, claiming his internships were of great benefit to those who took them up.

The advert produced by Leech's office and posted on the w4mp website says candidates would, "dependent on skills and suitability", be placed in departments focusing on casework, field organising, research and media, and diary, e-communications and fundraising.

Charles Glover, chief of staff of Leech's office, told the Observer the positions they made available, both in the constituency office in Manchester and in parliament, were not "real jobs" and did not require remuneration. However De Grunwald claimed Leech's advert suggested that the interns would be expected to do more than observe and pick up skills.

She said: "This latest advert suggests Mr Leech's office's are all over the place. The words 'intern' and 'volunteer' are used interchangeably.

"The internship is billed as a chance to "gain insight" – suggesting an observing role – but applicants are told they will "need to demonstrate team-working, computer and writing skills" and "an energetic can-do attitude".

"Why would they need those if they're only work shadowing and not doing any real, useful work?

"Most reasonable people now grasp the objections young jobseekers and campaigners have to unpaid internships – but for some reason, John Leech just isn't getting it.

"Having posted adverts for unpaid internships on nine separate occasions, he is by far parliament's most prolific employer of unpaid interns."

In May, new laws to outlaw unpaid internships that last longer than four weeks were proposed to parliament.

A motion proposed by Tory MP Alec Shelbrooke, which had crossparty support, would limit any unpaid work experience to four weeks. It is now up to the government to give the proposed law time in the Commons for it to be passed as legislation.

A Lib Dem spokesman said they had previously had six unpaid interns working at their headquarters but that this had been now reduced to two paid positions. He added: "John Leech and his office feel quite strongly about the benefits that he feels a number of interns who worked for him have enjoyed."

A spokesman for the websites w4mp said that it had added a note to Leech's advert making it clear that the position was a voluntary one and that there were "no set hours and responsibilities".