A senior Conservative peer has been acting as an adviser to a European lobbying firm for five years without declaring it to Parliament, an investigation by the Bureau of Investigative Journalism and The Independent has found.

Lord Plumb, the former president of the European Parliament, has been associated with the Brussels-based lobbyists Alber and Geiger since 2007.

But despite sitting on the Lords' EU Select Committee until May – which scrutinises draft EU law and suggests improvements – Lord Plumb does not declare his involvement with the firm in his entry on the Lords Register of Interests.

He is one of a number of senior members of the House of Lords not disclosing apparent outside interests.

They include Lord St John of Bletso, one of the 92 hereditary peers currently sitting in the House of Lords, who last year joined the "special advisory board" of a digital media firm, but this is not declared on the register.

Lord St John meanwhile sits on the House of Lords Communications Committee which is looking into the need for superfast broadband and has previously scrutinised TV advertising.

Lord Plumb: 'a simple farmer'

Under the House of Lords rules members are required to register all their relevant interests – both financial and non-financial.

"The key consideration in determining relevance in respect of both registration and declaration of an interest is that the interest might be thought by a reasonable member of the public to influence the way in which a member of the House of Lords discharges his or her parliamentary duties," the code says.

On his entry in the Register of Interests, Lord Plumb lists his only remunerated employment as "farming". But inquiries reveal that the former head of the National Farmers Union, who went into politics in the 1980s as a Tory MEP, has a more relevant interest which is not declared in the register.

He is named as a "team member" on Alber and Geiger's website, which describes the firm as a "political lobbying powerhouse".

The peer contributed to the EU Lobbying Handbook written by Andreas Geiger and published in 2007. A year later Mr Geiger described him in an interview as a partner in the new firm. In March this year Lord Plumb was appointed head of Alber and Geiger's new London office, according to the legal press.

Between 2007 and last May the peer sat on the Lords EU Select Committee, which scrutinises draft EU law and suggests improvements. He also sits on its sub-committee on the internal market, energy and transport.

Alber and Geiger describes its clients as "large companies, NGOs, and states that need our support in Brussels and in the European member states". Its entry on the European Commisson's transparency register says it lobbies on behalf of the kingdom of Morocco, the Bulgarian telecoms company Vivacom, the German packaging firm Papier Mettler and the port of Belgrade.

But when contacted by The Independent, Lord Plumb said he did not head the London office and it was "misleading to regard me as being or, having been, a senior partner" in the firm. He said he "did not object" to Alber and Geiger using his name to "drum up business" but insisted he did not need to register his involvement because he had "never been in employment, paid or unpaid", by the firm and did not have a financial stake in it.

However despite numerous requests to both Lord Plumb and Alber and Geiger, neither would say whether he had been remunerated for his work.

Andreas Geiger, from the firm, said: "Lord Plumb has of course no full-time engagement with our firm. He helps us in his professional capacity in an advisory role on Brussels-related matters."

Lord Boateng: 'a simple lawyer'

Detailed investigation of the interests of current members of the House of Lords suggests there is significant room for ambiguity in what is and what is not registerable – particularly in regard to what a member of the public would think may influence the manner in which a member of the Lords would act.

Lord Boateng, a former Labour minister and rising star under Tony Blair, and more recently the British high commissioner to South Africa, acts as international legal counsel to the consultancy DaMina Advisors, which claims to draw on "high-level political, legislative and government contacts" in order to advise companies with investments in Africa.

However, Lord Boateng does not declare his interest in DaMina despite frequently asking questions on Africa in the House of Lords.

A solicitor turned barrister by profession, Lord Boateng was one of the first three black MPs to be elected to the House of Commons and became the UK's first black government minister in 1997 when Tony Blair was elected. In 2002 he was appointed to the Cabinet as Chief Secretary to the Treasury before stepping down from Westminster to become high commissioner to South Africa. He was appointed to the House of Lords in the 2010 dissolution honours.

Contacted by The Independent, Lord Boateng said that he declared his profession as a barrister and legal consultant on his entry in the Register of Members' Interests.

But he added: "I am not currently required by the House rules to list my professional clients. My role with DaMina Advisors is as legal counsel advising them on legal and regulatory matters in relation to their African business."

He added that "any work for them is completely separate from my work in the House" and that he is "careful to avoid any actual or perceived conflict of interest".

Lord St John of Bletso

Another peer with undeclared interests is Lord St John of Bletso, one of the 92 hereditary peers currently sitting in the House of Lords.

In November 2011 Lord St John, who is also a Lord-in-Waiting to the Queen, joined the "special advisory board" of Mooter Media, an Australian digital media and online advertising firm. He is also an adviser to the technology marketing agency Mcdonald Butler Associates. Lord St John sits on the House of Lords Communications Committee.

Lord St John said both positions were not remunerated but he had asked for his name to be removed from the website of McDonald Butler. He added that he would speak to the Clerk of Parliaments to see if he was required to record the interests in the register. Under the House of Lords rules members are required to declare all their relevant interests – both financial and non-financial.

What does this mean for the future of the Lords?

The findings come at a time when the future of the House of Lords is under intense scrutiny. The Coalition Government has put forward ambitious proposals to reform the Lords to make it a predominantly elected second chamber.

Opponents of the plan say such changes would undermine the unique qualities of the current unelected house – with a variety of different outside interests represented.

A spokesman for the Lords said last night: "The House of Lords is committed to the highest standards of openness, transparency and accountability in regard to members' interests.

"Members have an obligation to keep their entry in the Register of Interests accurate and up-to-date. To enable members to comply with the requirements of the House's code of conduct there is a registrar of Lords' interests to advise members of their duties under the code, and an independent Lords Commissioner for Standards to investigate complaints of non-compliance with the rules."

Anything to declare? What the rules say

Under the rules, members of the House of Lords are required to register "all relevant interests".

Relevant interests may be financial or non-financial, and the key consideration in determining "relevance" is that the interest might be thought by a reasonable member of the public to influence the way in which a member of the House of Lords discharges his or her parliamentary duties.