The government has not demonstrated the necessity for its planned changes to judicial review, an all-party committee of MPs and peers said on Wednesday. And proposals by the justice secretary, Chris Grayling, called into question his very role.

In a measured and well-researched report, the joint committee on human rights said that Grayling had not made the case for his claim that challenges to public authorities in cases not involving immigration had "expanded massively" in recent years. Immigration claims are no longer heard in the high court, the committee pointed out, and when these cases are excluded from the figures the number of claims has remained remarkably steady.

Grayling ​modified some of his initial proposals in response to sharply-worded criticism from the judges, as I reported in February. But the MPs and peers were concerned that changes in costs rules would deter contributions by bodies that intervene in individual cases to assist the courts. And they said that restricting the availability of cost-capping orders until a later stage in the proceedings would undermine effective access to justice.

The committee reserved its most far-reaching criticism for Grayling's dual role as lord chancellor and secretary of state for justice. Until Lord Irvine was sacked by Tony Blair in 2003, the lord chancellor was a quasi-political figure with, as the committee put it, the "role of standing up within government for the interests of the justice system". Now, it said, the holder of that office was a "political minister in a government which has collective responsibility for its political views".

The committee noted that Grayling had written an article for the Daily Mail on 6 September 2013, the day on which his judicial review consultation was launched, suggesting that judicial review was being used as "a promotional tool by countless leftwing campaigners".

The committee said:

Such politically partisan reasons for restricting access to judicial review, in order to reduce the scope for it to be used by the government's political opponents, do not qualify as a legitimate aim recognised by human rights law as capable of justifying restrictions on access to justice, nor are they easy to reconcile with the lord chancellor's statutory duties in relation to the rule of law.

The MPs recognised that ministers had a legitimate interest in ensuring that judicial review was not abused in a way that incurred unnecessary public expense or delayed the decisions of democratically elected public bodies. Nevertheless, they continued,

the lord chancellor's energetic pursuit of reforms which place direct limits on the ability of the courts to hold the executive to account is unavoidably problematic from the point of view of the rule of law. Providing independent judges with the means to deal adequately with possible abuses is an important part of the constitutional arrangements … In our view, the government's proposals on judicial review expose the conflict inherent in the combined roles of the lord chancellor and secretary of state for justice.

The committee concluded that these issues should be considered by other Lords and Commons committees and recommended a "thoroughgoing review" of Grayling's combined roles.

Such criticisms were anticipated by parliament when it passed the Constitutional Reform Act 2005, which begins with fine words about the lord chancellor's "constitutional role in relation to the rule of law". The act also requires him and other ministers to uphold the independence of the judiciary. Grayling told the committee he took these responsibilities very seriously.

But there must be some doubt about whether these provisions could ever be used as the basis for a legal challenge. My own view is that the conflicts identified by the committee are inherent in the role of a secretary of state who, like any other politician, is seeking further advancement within government.

​It is significant that a well-respected cross-party committee of senior MPs and peers has drawn attention to the effects of politicising the role of lord chancellor. There is no question of going back to the pre-2003 arrangements, which were already showing strain at the time. But that makes it all the more important for parliament to give the most intense scrutiny to proposals that ​may further threaten the rule of law.