Other previously secret documents released to the tribunal on Tuesday show that MI5 was repeatedly warned by the Interception of Communications Commissioner’s Office [IOCCO] that it was breaching a code of practice on data collection that requires the agency to appoint a designated person independent of an investigation to authorise data collection.

The IOCCO found that MI5 had implemented independent oversight of data collection from people such as doctors and lawyers in professions with access to privileged or confidential information.

But in the majority of cases it found that “line managers are not independent from the operations for which they are granting authorisation or giving notices”. In December 2014, it urged MI5 to review its oversight measures. A year later it found that no progress had been made and reminded MI5 that the home secretary had said it was “imperative” that such independent oversight was achieved.

In Parker’s first letter to May, dated 19 March 2015, he said independent oversight of data collection would “cause significant disruption” and “reduce our effectiveness … It would increase the processing time for [data] requests because those taking the decisions would not be familiar with the relevant investigative context”.

He went on to suggest there was no public clamour for the change. “There does not appear to be a pressing litigation or reputational requirement to commit to make these changes now and we can therefore see no obvious gain in doing so,” Parker wrote. A month later Parker warned May that MI5 “will not hit your deadline” after she asked it to “strengthen the independence of authorisation for communications data”.