By Nick Assinder
Political correspondent, BBC News website
It is hard to think of any other event that could prove to be as damaging to Tony Blair as the arrest of his chief fundraiser Lord Levy.
Some believe Tony Blair may be the next person questioned
Indeed, in the wake of the arrest and release on bail of Lord Levy, the question dominating Westminster is whether the prime minister himself will now face police questioning.
And some are openly wondering whether the damage already done could be enough to bring down the prime minister.
Ministers and party officials are refusing to get drawn into commenting on the police inquiry, although it is being pointed out that all those involved have denied any wrongdoing.
Lord Levy has accused the police of using their arrest powers unnecessarily and said he is willing to fully co-operate with them.
But news of the arrest sparked crisis meetings in Downing Street and sent shockwaves throughout the Labour Party.
The reason is simple. This whole affair involves the prime minister directly.
Lord Levy - dubbed Lord Cashpoint because of his fundraising success - is not a minor party official, he is one of the prime minister's closest friends and tennis partner.
He was given two roles by Mr Blair - cash raiser and Middle East envoy - neither of which carried any direct party or Cabinet responsibility or accountability - he reported directly to the prime minister.
Lord Levy has denied any wrongdoing
Indeed, party officials including Treasurer Jack Dromey were kept in the dark about Lord Levy's activities in raising loans for the party.
And he has not been arrested in some trivial investigation, but as part of the ongoing police inquiry into whether laws have been broken in the so-called "cash-for-peerages" row.
So, Lord Levy is as close to Tony Blair as it is possible to be.
As a result, it seems impossible for the prime minister to be entirely insulated from anything that now happens to his friend.
And that has inevitably raised questions over whether the Metropolitan Police may yet seek to interview the prime minister himself.
Leader of the SNP Alex Salmond, whose party originally called for the inquiry, claimed "the waters are now lapping around the prime minister's feet".
And shadow home secretary David Davis said: "I'm quite sure Scotland Yard will have to question Tony Blair because he is at the top of the honours process. I imagine they will have to question him.
"The police are clearly taking this seriously, quite properly in my view. It
must be pretty worrying for the top ranks of the Labour party."
The sight of the British prime minister being questioned by police is likely to further erode the public's view of politicians and the political process, already at a low ebb.
It is certainly hard to see how it could lengthen the amount of time Tony Blair is to spend in Downing Street before stepping down.