When Theresa May published the draft Investigatory Powers Bill last Wednesday, Andy Burnham gave it a warm reception. "We support the government in their attempt to update the law in this important and sensitive area," the shadow home secretary told the Commons, adding that the legislation was "neither a snooper’s charter, nor a plan for mass surveillance".

There were some shadow ministers who believed the party should have adopted a critical stance towards the bill, which will force communications firms to store individuals' internet connection records (the URLs of the sites visted) for 12 months. Liberal Democrat leader Tim Farron accused the opposition of behaving like a "nodding dog". Now, in a letter to May, released to the New Statesman, Burnham has raised concerns over the legislation and demanded far stronger safeguards.

While emphasising that he wanted "work constructively" over the bill, he wrote to the Home Secretary: "I have now had the opportunity to study your proposals in detail and have taken advice from the Shadow Justice Secretary. This has given rise to concerns that the safeguards you are proposing are not as strong as it appeared when they were presented to the Commons."

Burnham echoed concerns raised by civil liberties campaigners over the so-called "double lock" for intercept warrants, warning that judicial authorisation would not be required. He writes: "[You] created the impression that both the Home Secretary and a senior judge would review the evidence. Indeed, you may recall that I asked you in the House about what would happen if there was a difference of opinion between the two."

He came close to accusing May of misleading MPs over the legislation. "On closer inspection of the wording of the Bill, it would seem that it does not deliver the strong safeguard that you appeared to be accepting. The current wording of the draft Bill requires the judge to review the 'process' undertaken by the Home Secretary in the same way applied to a judicial review: 'apply the same principles as would be applied by a court on an application for judicial review.' Legal advice we have sought confirms that the current wording does not deliver what we believed was being proposed in terms of the Home Secretary and Judicial Commissioner double-lock for warrant authorisation."

Burnham added that if Labour's "understanding was correct" it would look to amend the bill at Committee stage to ensure a genuine "double lock". Burnham also argued that the legislation "needs to include clearly-defined thresholds for access to internet connection records" and that the records should be limited to "police officers of a specified seniority". He concluded: "I believe you have produced a framework which has the potential to give the authorities the powers they need whilst also commanding public trust. But that will only be achieved by strengthening the safeguards in the areas I have identified."

Whether or not Burnham is successful, Labour's civil libertarians will be far happier with his new stance. The letter can be read in full below.

Update: The Lib Dems have been in touch and they aren't impressed. A spokesperson told the NS: "This is the latest flip-flop in the career of the most inconsistent politician in modern times. We won't put any stall by his letter as he might change his mind next week.

"The Liberal Democrats will keep making the principled case and calling for proper judicial authorisation."

Andy Burnham letter to Theresa May on draft Investigatory Powers Bill