The Muslim Council of Britain has warned Newcastle's Muslim players that wearing shirts carrying Wonga's sponsorship could infringe Sharia law.
Shaykh Ibrahim Mogra, assistant secretary general of the MCB, told the Independent:
"The idea is to protect the vulnerable and needy from exploitation by the rich and powerful.
"When they are lending and are charging large amounts of interest, it means the poor will have short-term benefit from the loan but long-term difficulty in paying it back."
"The Islamic system is based on a non-interest-based system of transaction," he added.
Mogra points out that former Tottenham striker Frederic Kanoute, a practising Muslim, was allowed to wear an un-branded shirt when playing for Sevilla, who were at the time sponsored by gambling firm 888.com.
"We've been following the controversy over the stadium naming rights and we're fans ourselves - football fans - and we totally get the passion that comes with football," said Wonga CEO and founder Errol Damelin.
"We listened to the fans - the fans want it to be called St James' Park, and we're delighted to be able to be part of solving that problem."
Unite, which has mounted a campaign against payday loan firms, attacked the deal, claiming it was being used to "normalise legal loan sharking".
Regional secretary Karen Reay said:
This is the day when Newcastle's owners sold this city's great footballing name for 30 pieces of tainted silver.
Payday lenders are preying on the poor and desperate in the North East, which has some of the highest levels of debt and borrowing in the country. Newcastle United is now being used by Wonga to normalise legal loan sharking.
Newcastle fans, who could be borrowing up to £325 per month just to get by, let alone get along to a game, will feel sickened that the club they love will now be associated with the extortionate rates of credit that make their lives a misery.
Unite urged the club's owners to reconsider the deal, warning it will "tarnish" the city's footballing and community culture.