As a general rule, it ill behoves one newspaper to attack another. But there are exceptions to every rule. The Guardian's behaviour this week, even by its own often disreputable standards over Israel, has been simply shocking.
In collaboration with Al Jazeera, it has presented a series of leaked memos written by Palestinian Authority negotiators with Israel. Not presented them so much as lobbed them, doused with petrol, into a tinderbox - and hoped for a result. There is nothing, of itself, wrong with the Guardian publishing its scoop; all serious newspapers relish scoops.
What is very wrong is the way the paper chose to present its story: the distortions, the bias, the agenda, the spin and the breathtaking arrogance of its handing down instructions to the Palestinians of how they should behave. Make no mistake: the Guardian's presentation was, as David Landau puts it, "intended to poison the Palestinians against their leaders". And to poison the world against Israel. Take the quote from Saeb Erekat, in which he was reported to have made an offer to Israel of "the biggest Yerushalayim in Jewish history". This was used to attack the Israelis for their intransigence.
"Israel spurned Palestinian offer of biggest Yerushalayim in history'' ran the headline. Nowhere was the preceding sentence from Erekat to be seen: "Israelis want the two-state solution but they don't trust. They want it more than you think, sometimes more than Palestinians."
Erekat himself was acknowledging the Israelis' desire for a solution. But because such a view does not fit with the Guardian's agenda, his words were brazenly distorted. The paper's editorial then attacked the Palestinian negotiators for being "craven", arguing that their apparent willingness to make concessions was a betrayal of the Palestinian people.
So it was hardly surprising, although still shameful, that on Wednesday it gave its main comment space to Hamas to threaten "practical measures" to "regain the initiative".
Hamas' practical measures need no elaboration. The Guardian crossed a line this week. It has not practised journalism but rather hardcore political activism, playing with people's lives.