Critics have also pointed out the "alarming" fact that a lobbying firm founded by the Tory peer, Quiller Consultants, works for private healthcare investor Circle Holdings, which backs a controversial US-EU trade deal - referred to as TTIP - that could "lock in the privatisation of public services, including the NHS".
Lord Hill has promised to sell his £375,000 stake in Huntsworth, a Brussels-based lobbying firm specializing in private healthcare that bought his firm Quiller in 2006. However, according to the register of interests in the House of Lords, he has not yet done so.
As the European Union's financial services commissioner, appointed by the UK government, he will represent City institutions and hedge funds in Brussels. Critics say this means Lord Hill will be representing firms like Lansdowne Partners, Odey Asset Management and BlueCrest Capital, who are heavily invested in private health firms like Circle.
Green MEP Molly Scott Cato, who sits on the European Parliament's ECON Committee which will scrutinise Lord Hill's work, told the Huffington Post UK: “This appointment would mean that a financial industry lobbyist would be in charge of financial regulation; it’s a clear case of poacher-turned-gamekeeper.
"As a founding director of Quiller Consultants, who are lobbyists working on behalf of financial interests, and with his well-developed contacts to the City of London, he would be in a key position to influence future financial market regulation."
"The revelation that he also has links to a multinational health firm which supports the controversial TTIP trade deal is equally alarming. This deal threatens to lock-in the privatisation of public services, particularly health services including the NHS."
On the blog: Len McCluskey - "No Government Has a Mandate to Allow the Irreversible Sale of Our NHS"
The Transatlantic Trade and Investment Partnership (TTIP) has garnered controversy due to the apparent secrecy of its negotiations and the fear that the NHS could be opened up to US companies in an "irreversible" privatisation process.
Unite general secretary Len McCluskey said in a statement: “Across the country people are calling on David Cameron to exempt our NHS from TTIP, which threatens to make the NHS sell-off irreversible. He responded by nominating a Lord with financial links to the very firms lobbying to keep it in.
"Now, Lord’s Hill’s appointment to a senior position in the Commission, where he can use his influence to get the deal agreed, is clearly compromised by this conflict of interest.
By nominating Lord Hill and then lobbying for this appointment, Cameron has once again gone to Brussels on behalf of his friends in The City, while turning his back on the groundswell of opposition which is building against this dangerous deal.
“The Tory’s Health and Social Care Act let private health firms, and the City investment firms who own them, loose on our NHS. Now David Cameron has helped these firms get a seat at the European Commission’s table."
The Huffington Post UK asked Downing Street for comment in response to the criticism of Lord Hill's appointment, but has yet to receive a response at the time of publication.
The Tory peer has been more warmly received by business groups. Simon Walker, director general of the Institute of Directors, said: "Entrusting this commitment along with financial services to a UK Commissioner is a clear demonstration that British influence in Brussels is far from waning."
“The torrent of financial sector regulation that has come from Brussels over the past 5 years has often been counterproductive," he added.
"Further liberalisation as a means of boosting business and economic growth is vitally important for both the UK and the EU as a whole."
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Syed Kamall, leader of the Conservatives in the European Parliament, recently poured scorn on the "myths' surrounding the controversial TTIP negotiations.
Blogging on the Huffington Post UK, he wrote: "If the TTIP negotiations cover issues other than trade, it becomes known as a "mixed agreement" and will have to be ratified by the British Parliament.
"Democratic oversight and transparency is a core shared objective of the parties and Members of the European Parliament across the political spectrum are closely monitoring the different stages of the process in order to inform our citizens and to engage them in the process."