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It takes one to know one.

True, Sir John Major is not the only former Tory leader familiar with being pressured, perhaps held hostage by the Eurosceptics in his party. Or indeed, the only Conservative leader ever to have been challenged by his party's preponderance to "bang on" (to use David Cameron's phrase) about Europe.

But arguably his experiences as the leader during bruising encounters with the "bastards" mean his words of warning might hold some value for the current prime minister.

For Theresa May, also an unflashy leader who was propelled to No 10 by a surprising political moment, Europe will be defining in a way no others could even have anticipated.

In Sir John's carefully calibrated speech tonight, there are plenty of messages for her, some of which may be welcome, some not.

First off, having campaigned to stay in the European Union, with sober warnings particularly about the consequences for the Northern Irish peace process, it's no surprise that Sir John says that in his view, Brexit will be a "historic mistake".

It is notable, although again not surprising, that he cautions that the UK will be a diminished diplomatic force in the world after we walk away from the EU, with a warning too that we will be less useful to our most important ally the US as a consequence.

Also, even as the PM who lived through the Commons trauma of trying to deliver the Maastricht Treaty, it is logical that he calls for Parliament to have a full role in shaping the negotiations over our place in Europe.

What may be harder for No 10 to dismiss is Sir John's obvious political concerns about how the public are being treated in the months after the referendum decision.

Despite insisting he has no desire to be in politics now, he makes very pointed criticism of the atmosphere around the debate, warning that voters are essentially being misled saying: "People have been led to expect a future that seems to be unreal and over-optimistic.

"Obstacles are brushed aside as of no consequence, whilst opportunities are inflated beyond any reasonable expectation of delivery."

In his first public comments since the vote, the former prime minister is offering what he describes as a "reality check".

And he sounds alarm bells too about the tone of the debate, saying Brexit's "cheerleaders" have shown "contempt" to Remainers, shouting down dissent "against our traditions of tolerance".

While he is not seeking to be unhelpful to the government, Sir John plainly has doubts about Number 10's handling of the process so far - the "rosy confidence" being offered to the British people.

And in the depths of his speech there is another warning for Theresa May about the Tory MPs she has worked so hard to keep on side - "today they may be allies of the prime minister, the risk is that tomorrow they may not".

Might Theresa May face her own "bastards" one day?

In recent weeks, with Theresa May determined to keep the Tory party together, and Labour struggling to stay united, the momentum has most certainly been with those celebrating our journey toward the exit door.

Ministers, even those who were ardent Remainers, privately sound increasingly optimistic about the prospects of doing a deal. But Sir John Major is not alone in having fundamental concerns. And his voice is harder for the government to dismiss, as they did Tony Blair a couple of weeks ago.

One senior figure even told me some of the talks behind closed doors have been a shambles, and raised concern that the government, all of us, are a long way from understanding the full implications of the decision.

Yet with almost the only political pressure on her coming from the right, Theresa May has decided to emphasise the opportunity, not the risks. The government is well aware that things could go wrong, but one minister told me "we all have to discover the reality together, when the rubber hits the road".