At the very moment the Football Association was finally opening St George's Park, centre for the sporting values modern football was always supposed to embody, one of England's great clubs, Newcastle United, unveiled a shirt sponsorship by the short-term, high-interest payday loan company, Wonga.

It emerged following late negotiations on Tuesday morning that the deal will now also include naming rights to Newcastle's stadium and, in a move designed to soften supporters' expected antagonism to Wonga, the ground will be called St James' Park again.

Both Newcastle and Wonga strove to portray the four-year, £24m deal as a great benefit to the team, youth academy and even the club's community work – but the outcry against it assailed St James' Park.

Stella Creasy, the Labour MP for Walthamstow, who has for two years led a campaign against Wonga and other "payday" finance companies she accuses of "legal loan sharking," said: "It is only through preying on families struggling to make ends meet that Wonga has made enough money to be able to sign this deal with Newcastle."

She was joined by local MPs Ian Mearns, Catherine McKinnell, Ian Lavery and the MP for Newcastle Central, Chi Onwurah, who also described Wonga as a "legal loan shark". Nick Forbes, the leader of Newcastle city council, called the sponsorship "disgraceful" and said Newcastle's deal to wear Wonga on its shirts would "undermine all the work we are doing to crack down on legal loan sharking".

Michael Martin, a lifelong Newcastle supporter and editor of the long-established True Faith fanzine, described the deal as "shameful," and said it "tarnishes the club's name, image and reputation".

With its jokey name and knowing marketing, Wonga has become the most visible company making "payday" loans, whose use is growing exponentially in the recession. The interest on Wonga's loans, one per cent a day, calculates over a year to an astonishing annual percentage rate (APR) of 4,214%.

Creasy's campaign calls on the government to cap interest, as is the practice in other European countries – France, for example, imposes a maximum of 21.63%; Germany 16.4%.

In the last financial year, to December 2011, Wonga more than tripled its income to £184m, and made £59m profit, from around 2.5m "payday" loans. The company is now planning an aggressive expansion of its operations; it already sponsors Championship club Blackpool, and Hearts in Scotland, but the £24m committed to Newcastle from 2013-17 will launch the Wonga name to a dramatically wider audience.

Concern about the financial difficulties suffered by already struggling people who take out high interest loans has been expressed by the Consumers' Association, Citizens Advice, and by R3, the national membership body for insolvency professionals. Lee Manning, the R3 president, responded to the news of Wonga's Newcastle sponsorship by saying "high-cost credit" adds to debt problems of people suffering financial difficulty, particularly in the north-east, where Newcastle is an iconic institution.

Official figures show the north-east has a higher personal bankruptcy rate than any other region in the country, that 26% struggle financially to make it to pay day, and an R3 survey found 70% "are concerned about their current level of debt". Loans such as those made available by Wonga are termed "payday" because they are often used by people struggling to make ends meet before they get paid. Referring to the Newcastle sponsorship, Manning said: "Wonga has chosen to target a region that has comparatively high numbers of people experiencing financial difficulty. Many of those seeking high-cost credit need professional advice for their financial problems, rather than accruing further debt."

Wonga itself argues that although the 4,214% APR figure is accurate, it does not fairly reflect its interest rate. The company points out its maximum loan term is 30 days – extended to 60 for late payers – and it charges 1% interest every day of a loan, not "compound interest" in which interest is charged on interest. Consumer credit law, though, requires all companies to state a typical APR, and on Wonga's average loan, £176, which is paid off on average after 16 days, at a cost of £34.14 in interest and fees, the APR does amount to 4,214%.

The company portrays its borrowers as young people exercising free choice, and says 84% pay their loans off on time or early, meaning 16% fall into arrears. However, an R3 survey last year found that 60% of people who took out a "payday" loan regretted having to do so, and 32% could not pay it off, so had to take out another one.

Derek Llambias, Newcastle's managing director who, with the club's owner, Mike Ashley, has made great strides to win over supporters since Newcastle's low point of relegation in 2009, depicted the deal as one whose cash would help Newcastle compete in the Premier League.

"Wonga's desire to help us invest in our young playing talent, the local community and new fan initiatives really impressed us and stood them apart from other candidates," he said. "I am delighted to welcome Wonga into the fold as our lead commercial partner."

It was not evident, however, that Llambias was quite ready for the level of dislike for Wonga, and the opposition which would greet the deal. Until Tuesday morning the agreement did not include the stadium, which was branded with Ashley's retail company, Sports Direct, until a naming rights buyer was found. Wonga's inclusion of the naming rights was then agreed, with the restoration of the St James' Park name seen as a PR move, because it is cherished by supporters.

Newcastle have also landed themselves in an already existing campaign specifically targeting Wonga's use of football for marketing. In July, delegates to the Football Supporters Federation's fans' parliament overwhelmingly passed a resolution which called on the football authorities to ban sponsorship by "payday" loan companies until they are subjected to tighter regulation and an interest rate cap.

The campaign, supported by Creasy, was begun by Bob Ward, a Northampton Town supporter, with an open letter to the Guardian in March. Signed by supporters from 18 clubs, it targeted Wonga's advertising on all Football League clubs' websites, a deal which has now ended. Ward said they plan to leaflet fans at Blackpool's matches with information about Wonga, and now intend to do the same at Newcastle's.

"We know where that £24m for Newcastle is coming from – many poor and vulnerable people paying appallingly high interest," Ward said. "Football clubs should promote good messages – perhaps responsible lenders like credit unions, not dreadful exploitation like this."

On the day the FA's national football centre was opened at last, a home for coaching and sporting development, Newcastle were telling a more complex story about modern football. Follow the money: high interest paid by people struggling to stay afloat, paid ultimately to millionaire footballers, who will wear Wonga on their shirts to promote more "payday" loans. Not, perhaps, the most inspirational message, for what was once the people's game.