It will never reach the PM. At least not to the extent that it implicates him enough in any way to resign.
I don't think it has to get that serious, look at the reasons the two met officers gave for resigning, not because they have been charged or officially implicated, but because this was now a 'distraction' to their important job and position.
The position of PM is all about appearance to the public (nowadays) giving the impression of being in control and making the right decisions for the country. If it is shown now that his judgement can be under question, due to the fact he was warned on multiple occassions that Coulson was trouble and not to employ him, then that hurts his authority going forward.
This would then start the murmmerings within the party, he could lose support and be told that 'resigning would be the best thing for everyone.'
Not saying it will happen, but it's a possibility.
Just be worried when the head of the 1922 comittee comes out and says 'Cameron has our full support, we are behind him all the way' ... when a politician says that it's usually a sign they are gone in under 2 days

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