Labour is stepping up the pressure on David Cameron following the resignation of Britain's top police officer, Sir Paul Stephenson.

The head of the Metropolitan Police stood down yesterday, citing speculation about the relationship between News International and the police force. The pressure on him grew with the revelation that he had employed the News of the World deputy editor, Neil Wallis. Notably, Stephenson directly referred to Cameron's relationship with the former News of the World editor, Andy Coulson:

Once Mr Wallis's name did become associated with Operation Weeting, I did not want to compromise the Prime Minister in any way by revealing or discussing a potential suspect who clearly had a close relationship with Mr Coulson. I am aware of the many political exchanges in relation to Mr Coulson's previous employment -- I believe it would have been extraordinarily clumsy of me to have exposed the Prime Minister, or by association the Home Secretary, to any accusation, however unfair, as a consequence of them being in possession of operational information in this regard. Similarly, the Mayor. Because of the individuals involved, their positions and relationships, these were I believe unique circumstances.

On the Today programme this morning, the shadow home secretary Yvette Cooper said that this raised questions about Cameron's "continued silence" on the matter. Here's the key section of the interview:

Cooper: It was interesting what Sir Paul said yesterday -- that one of the reasons he clearly felt he could not tell the Home Secretary, the mayor, Downing Street about that contract that he had with Neil Wallis, the former deputy editor of News of the World --- he couldn't tell them because of the relationship between the Prime minister and Andy Coulson. That seems to me to be unprecedented. I cannot think of any case where the commissioner could not tell the Home Secretary because he was worried about the Prime Minister's relationship with somebody involved in the criminal investigation.

Interviewer: To be clear, this resignation statement, he says "I did not want to compromise the Prime Minister in any way by revealing or discussing a potential suspect who clearly had a close relationship with Mr Coulson. But why would that have compromised the Prime Minister?

Cooper: Well, this is obviously Sir Paul's judgement --

Interviewer: Can you explain to us how that could be? It's difficult to know why it would compromise the Prime Minister. What are the options?

Cooper: I don't know the details of what it is Sir Paul knows about the ongoing investigation, what the role of Andy Coulson is. But as you'll know, the Prime Minister is obviously continuing to see Coulson, he invited him to Chequers some time after his resignation, so he has obviously continued to be in touch with Andy Coulson. So there are clearly questions I think about Andy Coulson's role in all of this and about the Prime Minister's judgement in appointing him and in continuing to keep that relationship up. So it does raise concerns. If the Met commissioner himself thought that relationship -- that compromised relationship -- prevented him from telling the Home Secretary what was happening, talking to her about operational things, but also maintaining the Home Secretary and the mayor's confidence in the on-going work of the Met and how they were handling a difficult situation -- that puts the Met commissioner in an extremely difficult situation.

Cameron is currently on a trade visit to Africa, a trip which he has cut from four days to two. However, his absence at this critical time looks strange to say the least. He has so far ignored the serious questions that his relationship with Coulson raises, except to say that if he was misled by Coulson, then so were police and parliament. Stephenson's comments -- while certainly not notable for their clarity -- seem designed to put the pressure back on Downing Street. This is potentially very damaging for Cameron: he will not be able to delay providing answers for much longer.

UPDATE: Cameron has rejected Stephenson's comparison between his hiring of Wallis and Cameron's hiring of Coulson. Speaking at a press conference in South Africa, he said:

I think the situation in Metropolitan Police service is really quite different to the situation in government, not least because the issues that the Metropolitan police service are looking at and the issues around them have had a direct bearing on public confidence into the police inquiry into the News of the World and indeed the police themselves.