The spectacle of a Conservative Party treasurer hawking access to the Prime Minister in return for "premier league" donations to the Tory coffers is deplorable. But it is hardly surprising. And until party political funding is reformed, and big-money donations outlawed, such shady practices will inevitably continue.

That is not to say that Peter Cruddas's actions are excusable. His shameless claims about the returns to be had on donations of £100,000 or more – of views fed into the Downing Street policy unit, of dinners with David Cameron and his wife in the private apartments in Downing Street, of "awesome" benefits to the donor's business – are a stain on the British political system. Mr Cruddas may have hoped his swift resignation might nip the scandal in the bud. It will not. Neither do his assertions that "there is no question" of donors being able to influence policy or gain undue access even begin to undo the damage wrought by his oily sales pitch. The Prime Minister is promising an inquiry. So there should be. But that is not enough.

Before the election, David Cameron made a powerful speech warning that corporate lobbying was "the next big scandal". He identified that such activities feed voters' suspicions of "money buying power, power fishing for money, and a cosy club at the top making decisions in their own interest". Absolutely right. But what has he done about it since? Not much.

It is not as if Mr Cruddas's behaviour is the first hint of impropriety in Westminster. The Defence Secretary, Liam Fox, was forced to resign last year because of the wholly inappropriate access accorded to his friend Adam Werritty. An investigation by this newspaper then added to the sense of a back door to political influence, revealing Bell Pottinger public relations executives boasting that clients' "messages" would get through to the top of Government. Meanwhile, the phone-hacking inquiry has shone a light on the close, often social, links between politicians and Rupert Murdoch's newspaper executives.

It can only be hoped that the embarrassment over Mr Cruddas will finally force Mr Cameron's hand. There are three priorities. As Prime Minister, his overwhelming responsibility is to put paid to the highly damaging suggestion that the British Government is for sale. That means immediately publishing complete lists of all donors' access and any input into policy discussions.

Second, Mr Cameron must keep his promises on lobbying. True, the Coalition is now, finally, moving forward with plans for a public register of lobbyists. But the current proposals go nowhere near far enough. The register must include not only the identity of lobbyists themselves, but also full details of who they talk to and what about. The rules must also apply to all who are paid to lobby, not only those working for third-party agencies.

Third, and most importantly, the Prime Minister must make radical reforms to the way political parties are funded. All three main parties pay lip service to reform, but none has been willing to move beyond narrow self-interest. Spurred on by the appalling behaviour of Mr Cruddas, Mr Cameron should take a lead, putting his full weight behind Sir Christopher Kelly's recommendations that donations be capped at £10,000 per year and that parties be largely funded by the state. The electorate may baulk at a plan to subsidise politics, particularly when money is so tight. It is up to Mr Cameron to make the case that the 50p per-voter per-year levy estimated by Sir Christopher is worth every penny.

The Coalition has trumpeted its ability to make hard choices on the economy. It is time to take equally tough decisions elsewhere. No more excuses. Mr Cameron must live up to his rhetoric, make amends for his treasurer, and do what he can to clean up politics.