Prime Minister David Cameron is facing renewed questions over the phone-hacking scandal after the head of the Metropolitan Police resigned.
Sir Paul Stephenson was criticised for hiring ex-News of the World executive Neil Wallis as an adviser.Sir Paul said his integrity was intact, but referred to the PM's former aide Andy Coulson. The ex-NoW editor and Mr Wallis have been questioned by police.
Home Secretary Theresa May said no pressure was put on Sir Paul to quit.
But Labour shadow home secretary Yvette Cooper said the link between Sir Paul and Mr Wallis hindered Sir Paul's work.
Meanwhile Rebekah Brooks, the former News International chief executive who had been editor of the News of the World when the alleged hacking took place, was arrested and released on bail on Sunday as part of the investigation.
The questions threaten to overshadow the PM's trade visit to Africa - which he has had to cut from five days to two.
Potential suspect
At the weekend, Sir Paul was criticised for having hired former News of the World executive Neil Wallis as an adviser.
Mr Wallis was arrested and released on bail on Thursday on suspicion of conspiring to intercept communications.
In his resignation statement, the outgoing Met Police chief suggested that Mr Cameron's relationship with Mr Coulson, his former Downing Street director of communications, had caused him difficulties.
Mr Coulson, who resigned from the No 10 job and earlier as News of the World editor over the scandal, was subsequently arrested and bailed over his alleged involvement in paying police officers and phone hacking.
In his resignation statement, Sir Paul said: "Unlike Mr Coulson, Mr Wallis had not resigned from News of the World or, to the best of my knowledge been in any way associated with the original phone hacking investigation."
He went on: "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."
The shadow home secretary said if the Sir Paul felt that the "compromised relationship" between the prime minister and Mr Coulson "prevented him from telling the home secretary what was happening" and discussing operational matters with the home secretary, it meant the Met commissioner had been put in "an extremely difficult situation".
Ms Cooper said there were questions about Mr Coulson's role in the phone-hacking scandal and "the prime minister's judgement in appointing him and continuing to keep that relationship up".
The home secretary told BBC Radio 4's Today programme that both she and the prime minister had been clear that "the Metropolitan Police must investigate all allegations and investigate all evidence and take it as far as it goes".
"If the Metropolitan Police found, at any stage, that they have a potential conflict of interest, I think it's right for them to be transparent about that," she said.
BBC Radio 4's chief political correspondent Norman Smith said Sir Paul seemed to seek to push the focus back onto Downing Street by highlighting the prime minister's decision to recruit Mr Coulson, a move that has infuriated No 10.
He said it was Mr Cameron's links to Mr Coulson which still threaten to cause the most damage to the prime minister over the hacking scandal.Downing Street insist their hands are clean and that the prime minister only found out Sir Paul's plans half an hour before he made his televised resignation statement.
Our correspondent says it was clear that there was immense political pressure on Sir Paul after the Mayor of London, Boris Johnson, told him that his confidence in him had been severely shaken.
Aides to the mayor say while Mr Johnson did not march Sir Paul to the door with a gun to his head, it was made clear that it would be extremely difficult for Sir Paul to win back the mayor's and the public's confidence, our correspondent said.
One aide to the mayor said: "There was no way back."
David Cameron said he respected and understood Sir Paul's decision to resign as head of Scotland Yard.
He then urged the Metropolitan Police to continue to do "everything possible" to proceed with the investigation.
'Untouchable' The prime minister is currently in South Africa on a trip planned to boost trade with emerging African economies.
The BBC's deputy political editor James Landale said the trip had been cut back from five days to two so that the PM could focus on the hacking row after being criticised by MPs for being in Afghanistan when the scandal initially broke.
Number 10 said the trip had been shortened "simply because the prime minister has other things he wants to be focused on".
The home secretary is set to make a statement to the House of Commons later over the links between the Met and News International.
And Labour leader Ed Miliband is to blame the scandal on a culture of "irresponsibility" that also led to the banking crisis and MPs' abuse of their expenses.
In a speech later, he will say that all three episodes were caused by some of the most powerful people in society thinking they were "untouchable" and could act as they pleased.
