The RSPCA brought what it said was the first successful corporate prosecution against a hunt, the Heythrop Hunt in Oxfordshire, after independent monitors filmed its members killing a fox in contravention of the Hunting Act 2004.
Judge Tim Pattinson, presiding over the case, criticised the RSPCA for spending more than £320,000 in bringing the prosecution and said that the public might feel the money could be "more usefully employed".
A letter to the commission from the group of politicians, which includes the Conservative peer Lord Heseltine, the Tory MP Simon Hart, the Labour MP Kate Hoey and the Liberal Democrat MP Mark Williams, said trustees at the RSPCA had failed in their "duty of prudence" to the charity and its funds.
They said the money spent on bringing the prosecution was "staggering" and called on William Shawcross, the chair of the commission, to investigate.
The group cites Judge Pattinson’s comments in its letter, which says: "We believe that this staggering expenditure constitutes a clear breach of the duty of prudence by the trustees of the RSPCA in that it cannot possibly be argued that charitable funds and assets have been used reasonably."An RSPCA spokeswoman said the charity and its trustees had acted within its charitable objectives and procedures and in line with charity law.
She said the Heythrop Hunt case was pursued on the basis of "clear and explicit evidence of breaches of the law".
She said: "The Hunting Act 2004 is an act passed by the very parliament in which the MPs who have criticised us serve. In the RSPCA’s view, the laws of this land relating to animal welfare apply to everyone equally – nobody is above the law.
"It is clearly in the public interest that the law is seen to be upheld and that the animal welfare aims of the ban are achieved.
A spokeswoman for the Charity Commission said Shawcross would respond to the complaint in due course.
"Undertaking such prosecutions is in furtherance of the RSPCA's charitable objects and is made clear to the public on their website," she said. "It is for trustees to consider the matter of bringing prosecutions in accordance with these duties and any other requirements which might apply, and for them to consider the issue of costs."
The RSPCA has set up a ‘fighting fund’ to pursue private prosecutions and judicial review applications.