The deal, estimated to be worth £230m, will see Bain take a majority stake of 80% in Plasma Resources UK (PRUK), with the government keeping a 20% share.
This includes an upfront cash payment of £90million and a deferred payment expected to be worth £110m in 5 years’ time. Bain have also agreed to invest a further £50 million in the firm to create a "UK Life Sciences Champion".
Former health minister and foreign minister Lord Owen said: "It's hard to conceive of a worse outcome for a sale of this particularly sensitive national health asset than a private equity company with none of the safeguards in terms of governance of a publicly quoted company and being answerable to shareholders.
"Private equity has a useful function, as I saw in years past on the advisory board of Terra Firma, but Bain Capital should not have been chosen for this sale. Is there no limit to what and how this coalition government will privatise?"
Labour MP Valerie Vaz, who serves on the Commons Health Committee, told the Huffington Post UK: "It is unclear why Plasma Resource UK should be sold.
"It is important the NHS controls vital services such as plasma products. The Government has learnt no lessons from the use of contaminated blood in the 1970s and 1980s whose victims are still seeking justice."
Unite union head of health Rachael Maskell said: "This news is the thin end of the privatisation wedge. Everything we have warned about for the last two years is coming true – that this government wishes to sell-off large swathes of the NHS to profiteering private companies.
“It is wrong that this announcement was made just as MPs are about to take six weeks holiday which won’t allow parliamentary scrutiny of this development.”
Ministers were reportedly apprehensive about the prospect of selling PRUK to a foreign private equity group, but concluded it represented the best value-for-money.
When the deal was announced, health minister Dr Dan Poulter said: “This deal will ensure that patients will have access to high quality plasma products for years to come and it is good news that Bain are investing in medicine and the life science industry in the UK.
Bain developed a political notoriety during the 2012 US Presidential Elections as Mitt Romney’s experience at the company’s helm led to him being accused of being a “job destroyer”.
PRUK has annual sales of around £110m and consists of two companies, Bio Products Laboratory (BPL), which is staffed by around 200 people in the UK and over 1,000 in the US. The other company is DCI in America, which collects plasma from American donors and sends it to BPL where it is separated into blood proteins, clotting factors and albumin for supply to NHS hospitals.