Senator Dianne Feinstein is a former mayor of San Francisco who has represented California in the US Senate for more than 20 years. But a stereotypical west coast Democrat she most certainly is not. She supported the Iraq war and co-sponsored a 2002 law banning entry to the US from nations that sponsor terrorism. She has defended the sweeping detention and intercept powers in George Bush's Patriot Act. In 2007 she backed immunity for telecoms companies which allowed the National Security Agency to eavesdrop on international phone calls. Since becoming chairman of the Senate intelligence committee – the key job in the US security oversight process – she has consistently defended the NSA against the issues raised in the Edward Snowden files. Ten days ago she again gave another trenchant defence of the NSA's data trawling.

This week, however, Ms Feinstein did a spectacular about-turn. She called for a review of all US intelligence surveillance programmes. Not bits of them. Not this or that programme. All of them. The tipping point was the revelation that the NSA has been monitoring the calls of friendly world leaders such as Angela Merkel. Mrs Merkel's calls had been monitored since 2002, three years before she became German chancellor, the senator confirmed. And no one had thought fit to tell President Obama.

It is worth reading what Ms Feinstein said this week. "It is abundantly clear," she said, "that a total review of all intelligence programmes is necessary so that members of the Senate intelligence committee are fully informed as to what is actually being carried out by the intelligence community." To repeat: these are the words of someone who has staunchly defended the agencies, their powers and their work throughout the recent past. They are the words of someone who takes her oversight function seriously. And they are also truly devastating words.

Her change of heart should be taken very seriously. Senator Feinstein has not suddenly become an anti-government agitator. She is still committed to the agencies, their powers and their work. She continues to defend the NSA's sweeping collection of phone records, which she believes is a lawful process. But the revelation that the US has been routinely monitoring the communications of at least 35 world leaders, including Mrs Merkel, has led her to believe that all US intelligence surveillance programmes need to be comprehensively reviewed. Note the words once more: "Congress needs to know exactly what our intelligence community is doing. To that end, the committee will initiate a major review into all intelligence collection programmes."

Britain's politicians should take careful note of what is happening in Washington – and in a succession of European and other capitals too, most recently in Madrid. Responsible investigative journalism has unearthed a situation that the oversight system had no idea was taking place. The right response to that is the one that Senator Feinstein, in common with many other US legislators including the drafter of the Patriot Act, has now embraced. The NSA was not under proper control. It has extended its reach too far. It has misled the politicians. The systems of oversight and law therefore need to be re-examined. The problem is the surveillance programmes, not the journalism that has told the story.

Britain's parliamentarians have not yet measured up. It is time they did. Like the Americans, they need to focus on the message, not the messenger. They need to use their powers – they are there to be used – to probe and question the agencies and the oversight regime. They need to understand better, because Britain, like the US, needs a better balance between security and liberty in an era transformed by technologies that most politicians barely know exist. They need to look beyond Whitehall to Europe and the US. Look at the remarks of Senator Feinstein. She has set up a review of all intelligence surveillance programmes. If she can, why can't we?