Lawyers for Michael Shields, the Liverpool football fan serving a 10-year sentence for the attempted murder of a Bulgarian barman in 2005, have accused Jack Straw of misunderstanding the law.

As part of their attempt to persuade the Justice Secretary to grant Shields a free pardon, the lawyers point out that Mr Straw claims to be following a test laid down by the High Court - that a pardon must not be granted unless a convicted prisoner is "morally and technically" innocent.

But the lawyers say that the test of moral and technical innocence is the Justice Secretary's own policy, inherited from his predecessors, and not a test that Shields's lawyers had recommended to the court.

"It is not a legal test and it does not imply an impossibly high hurdle. The pardon exists as a backstop to prevent injustice," the lawyers say. "It is a flexible prerogative power to be adapted to the needs of the particular case."

In their view, the phrase "moral and technical innocence" merely indicates that the power operates to rectify real injustice rather than mere technical defects in the legal process. 

"This is what we (correctly) asserted to the High Court.  The judges adopted our arguments.  They did not have jurisdiction to lay out a legal test for the Secretary of State to follow, nor were they invited to by either party, nor did they purport to. 

"Indeed quite the opposite; they expressly asserted that it was not for them so to do," the lawyers say in a further submission to Mr Straw.

"It is an error for the Secretary of State to suggest that he is merely following the High Court ruling or that he is acting in a way proposed by those acting for Mr Shields."