Handling of Andy Coulson's appointment was either naïve or negligent

The relationship between a party leader and the man (it's almost always a man) who speaks for them is one of the most intimate and sensitive in politics. They must understand each other completely. They need to have a mutual grasp of the political challenges they face and share an instinctive sense of how to respond to events. And above all, they need to trust each other implicitly.

Historically the most successful prime ministers have enjoyed that level of trust – Thatcher, Blair, Wilson, Macmillan. The failures have not – Major, Brown, Eden. It's not a guarantee of electoral success but its importance cannot be overlooked.

As David Cameron has acknowledged, the decision to employ Andy Coulson was his and his alone. What he has so far failed to admit is that it was a judgement call – and he got it wrong. He may not have known the full facts – including the extraordinary revelation that Coulson was still receiving large amounts of News International cash at the time of his appointment and for months afterwards – but far from mitigating his error it only makes it worse.

Alarms bells should have rung loudly in Cameron's head. If they did not, he was naïve. If they did, and he chose to ignore them, he was negligent. Either way, it was a very poor reflection on the judgement of a man aspiring to be prime minister, a job that depends on gut instinct as much as shrewd calculation if it is to be done well.

The fact is that almost anybody with a long career in journalism is going to have some kind of baggage. Common sense would suggest that the former editor of a tabloid like the News of the World is going to have more than most.

That is not to say that Andy Coulson was dodgy by default. It is certainly not to suggest that he didn't have the qualifications to make an excellent communications director. But it is to assert that due diligence called for some searching questions to be asked. It also required a spirit of openness on the part of the candidate for the job. Trust is a two-way relationship. Before even considering such a role any potential spokesman needs to say to himself: "I'm probably going to have to help dig this person out of some deep trouble somewhere along the line. Am I sure I'm not going to drop him into any myself?"The full and frank discussion between Tony Blair and Alastair Campbell involved past alcoholism, a nervous breakdown and even some youthful porn writing. So you don't have to have lived the life of an angel to qualify. You just have to be honest about your past. I'm sure Ed Miliband has had a similar conversation with his spokesman, Tom Baldwin, who is also a former News International employee. At least I very much hope he has.

In Coulson's case the discussion would have needed to cover the kind of things he got up to, or was party to, as a tabloid journalist and then editor. It should also have included his financial affairs.

Senior figures in the media are paid a lot more than party spokesmen. They usually have to take a hefty pay cut to take a political job. Yet it seems that Coulson got a very generous deal from the Tory party. Eyebrows were raised at the time about his salary, reported to be in the region of £275,000 a year. If he was also receiving hundreds of thousands of pounds, a car and other benefits from News International as part of his redundancy agreement, he should have told his new employers. And they should have told him to go away and renegotiate it.

We don't know what discussions he did have with Cameron or anybody else. We do know that in public he was less than straightforward about his income. Tom Watson, the Labour MP on the media select committee was characteristically precise and to the point when he asked Coulson, "...so your sole income was News International and then your sole income was the Conservative party?" Coulson's reply was an unambiguous "yes".

In fact we don't need to know what discussions took place prior to the appointment in order to know that the requisite basis of trust was absent from the outset. The Prime Minister may be naive sometimes but he is no fool. If Andy Coulson had told him that he had an ongoing financial relationship with News International then Cameron could not have turned a blind eye to that. He wanted a man who understood how NI worked and could help get him the kind of positive coverage both Thatcher and Blair had enjoyed from that quarter. He didn't want a man who could be open to the charge of still being in the pay of Rupert Murdoch.

Party leaders, far less prime ministers, do not need to know the ins and outs of every detail before they make a decision. They couldn't function if they did. What they do need is an instinctive grasp of the big questions that must be asked and answered before they give their judgement. In the case of Andy Coulson it was pretty obvious. "Is there anything in your past or present relationship with News International that could embarrass either me or my party?"

If Coulson had answered that one with an unambiguous "yes" he would have saved everybody, including himself, a lot of trouble.

Lance Price is a former media advisor in 10 Downing Street. His book, Where Power Lies – Prime Ministers v The Media, is published by Simon & Schuster