An independent review to consider the operational case for bulk powers proposed in the Investigatory Powers Bill has been signed off by the home secretary.

Theresa May's offer was made in a letter to shadow home secretary, Andy Burnham, which he revealed in the House of Commons on Tuesday.

Burnham said: "I am pleased that she has agreed in the letter to look at having a review and has approached David Anderson QC to lead it. The Opposition strongly welcome that development, which we believe will build trust and support behind the bill." Anderson is the Independent Reviewer of Terrorism Legislation, tasked with reviewing the operation of the UK's anti-terrorism laws.

The government needs the votes of Labour's MPs for the Investigatory Powers Bill to be sure to pass. After initially offering rather uncritical support for the Snoopers' Charter, Labour later backtracked somewhat and highlighted areas that it said were problematic. One such issue was that "bulk personal datasets and communications data are overly broad."

Burnham hasn't published the letter from May, and—at time of publication—he was yet to offer further comment when quizzed by Ars.

Similarly, when we asked the home office for more information, it simply said: "The home secretary has always been clear she will listen to the constructive views of politicians from all sides of the House to ensure the passage of this important bill. The government will be bringing forward amendments at report stage.”

However, a story on the BBC contains the following unsourced details: "The independent review will be established under David Anderson once he has selected two experts with security clearance to support him on technical and legal issues. He will report by the summer in time for the committee stage of the bill in the House of Lords."

It is not clear to what extent the UK government will follow any of his recommendations, however.

On Tuesday, Burnham revealed to MPs a further attempt by May to win Labour's support: "The home secretary’s letter contains a suggestion that she will change the bill to ensure that investigatory powers cannot be used to monitor legitimate trade union activity. That is a major concession—historic, even—and I am certain that it will go a long way to reassuring opposition members."

However, he went on to emphasise that on their own, these moves were not yet enough: "There is still a considerable way to go before the Investigatory Powers Bill becomes acceptable, but this letter shows that the home secretary is listening, which bodes well for the rest of the bill’s passage."

That hints that Burnham is likely to capitulate on the Snoopers' Charter provided the UK government throws in a few more concessions that will allow Labour to claim the revised legislation as a "victory" of sorts.

Updated @ 15.17pm BST, May 26: Andy Burnham has published on his blog a letter to the home secretary in which he lays out five additional changes that he wants made to the Investigatory Powers Bill in order for it to be "acceptable" to the Labour Party.

If these are not forthcoming, Burnham says "we will table strong amendments in each of the remaining five areas. If these are defeated and no progress is made, we will be unable to support a timetable that puts this bill into law by the end of this year."