David Cameron will be forced to explain damaging new revelations today that have dragged him deeper into the phone-hacking scandal.

It emerged last night that Neil Wallis, the former News of the World deputy editor who was arrested last week, worked for the Conservative Party before last year's election. He gave "informal" advice to Andy Coulson, his former boss at the NOTW, who resigned from the paper over the hacking affair but was later appointed Mr Cameron's director of communications.

In a second blow to the Prime Minister, it was revealed that his chief of staff, Ed Llewellyn, had appealed to Scotland Yard not to mention hacking during a Downing Street briefing last September, four months before Mr Coulson quit his No 10 post. Labour said the disclosure showed Mr Cameron could not do his job properly because of the cloud cast by the hacking controversy.

Mr Cameron returned last night from a trip to Africa he was forced to cut short by a growing crisis which some Tory MPs fear is in danger of destabilising his premiership.

Loyalists believe the Prime Minister looks increasingly isolated and are concerned that cabinet members, including the Chancellor George Osborne and the Tory chairman Baroness Warsi, have failed to rally behind him while he has been away. But one backbench leader said: "The feeling is that this is a crisis of his own making – he employed Andy Coulson."

The Prime Minister's plan to go on the offensive today during a Commons statement on the affair suffered a setback with the disclosure that his party had links to two people arrested during the current police investigation – Mr Coulson and Mr Wallis.

A Tory spokesman said: "We have double-checked our records and are able to confirm that neither Neil Wallis nor his company has ever been contracted by the Conservative Party, nor has the Conservative Party made payments to either of them. It has been drawn to our attention that he may have provided Andy Coulson with some informal advice on a voluntary basis before the election. We are currently finding out the exact nature of any advice."

The Tories insisted that neither Mr Cameron nor any senior member of the party's campaign team were aware of Mr Wallis's involvement until this week. It is believed the advice was given on a one-off project during 2009. Ed Miliband will quiz Mr Cameron over the precise nature of Mr Wallis's role and over his chief of staff's apparent attempt to insulate the Prime Minister from the hacking scandal.

Mr Llewellyn has already been accused of not passing on to Mr Cameron warnings from senior Liberal Democrats and newspaper executives about appointing Mr Coulson after last year's election. Yesterday it emerged that Mr Llewellyn sent an email to John Yates, the former Metropolitan Police Assistant Commissioner, last September, saying he "would be grateful" if hacking were not raised by him during an imminent briefing on national security. "I am sure you will understand that we will want to be able to be entirely clear, for your sake and ours, that we have not been in contact with you about this subject," Mr Llewellyn wrote.

The briefing was held shortly after allegations in The New York Times that Mr Coulson knew about hacking while he was NOTW editor and "actively encouraged" it, claims he strongly denies.

Downing Street defended Mr Llewellyn, saying he cleared his request with Jeremy Heywood, the permanent secretary at No 10. Cameron aides said the plea was nothing to do with Mr Coulson but reflected a desire that politicians should not be involved in operational police matters.

However, Mr Llewellyn's request appears to have been in the mind of Sir Paul Stephenson, the Met Commissioner, when he decided last week not to tell Mr Cameron or the Home Secretary Theresa May that Mr Wallis had been employed as a PR adviser to Scotland Yard.

Mr Yates confirmed to the Home Affairs Select Committee that Mr Llewellyn made the request. "Ed for whatever reason – and I completely understand it – didn't think it was appropriate for him, the Prime Minister or anyone else in No 10 to discuss this issue... and [said he] would be grateful if it wasn't raised."

Sir Gus O'Donnell, the Cabinet Secretary, dismissed a complaint by the Labour MP John Mann that Mr Cameron had breached the ministerial code by meeting James Murdoch and Rebekah Brooks last Christmas while the Government was considering News Corporation's bid for full control of BSkyB.

Sir Gus O'Donnell, the Cabinet Secretary, said after studyng the exchange of emails that Mr Llewellyn "acted entirely properly."

Was he compromised? Cameron and the Murdoch empire

2007:

July Andy Coulson is appointed director of communications to Opposition leader David Cameron, some seven months after his resignation as editor of the News of the World. The appointment is reportedly made following recommendations from Rebekah Brooks, editor of The Sun, and George Osborne.

2010:

24 February Commons Media Committee accuses News International executives of "collective amnesia" concerning voicemail hacking and concludes it is "inconceivable" that managers at the paper did not know about the practice.

April (date not specified) Cameron meets Rupert Murdoch, and according to Downing Street, they hold a "general discussion". Neil Wallis, former deputy editor of the NOTW, provides "informal advice" to his old boss, Andy Coulson, prior to the general election. The Conservative Party last night confirmed the arrangement but said it had never employed or paid Mr Wallis.

12 May David Cameron becomes Prime Minister.

May In his first three weeks as PM, Cameron holds five meetings with News International – Rebekah Brooks at Chequers; Dominic Mohan, editor of The Sun, for a general discussion; the News International summer party; James Harding, editor of The Times, for an interview; Times CEO Summit for a speech.

14-16 June First disclosure of the Murdochs' plans to take full control of BSkyB. The broadcaster's board asks for at least 800p per share.

June The Prime Minister attends The Sun Police Bravery Awards reception and dinner awards ceremony and meets Dominic Mohan for a general discussion and Colin Myler, editor of the NOTW, for general discussion.

July Rebekah Brooks visits Cameron at Chequers.

August Cameron meets John Witherow, editor of The Sunday Times, for a general discussion.

1 September The New York Times publishes an article alleging widespread knowledge of phone hacking at the NOTW, including interview with former reporter Sean Hoare alleging that Andy Coulson knew of the practice.

September Ed Llewellyn, Cameron's chief of staff, turns down the offer of a briefing from Met police Assistant Commissioner John Yates about a review of the phone-hacking investigation. Llewellyn, whose boss appears to be at risk of being compromised by his employment of Coulson, says he would be "grateful" if the matter was not raised.

September Cameron meets James Harding, Dominic Mohan, Rebekah Brooks and John Witherow separately at the Conservative conference; he also attends the NI reception at the event.

11 October An alliance of media companies opposed to the News Corp/Sky deal – including BT, Channel 4 and the publishers of The Guardian, Daily Mirror, Daily Mail and Daily Telegraph – writes to Business Secretary Vince Cable saying the deal could have "serious and far-reaching consequences for media plurality".

9 October Rebekah Brooks attends PM's 44th birthday party at Chequers.

October James Murdoch and his wife Kathryn visit PM's country retreat.

November Coulson is interviewed as a witness by Metropolitan Police detectives investigating the phone-hacking allegations. He is not cautioned or arrested.

3 November News Corporation notifies European Commission of its intention to acquire the shares in BSkyB that it does not already own.

4 November Vince Cable intervenes in proposed bid to gain full ownership of BSkyB, ordering media regulator Ofcom to review deal on the grounds of "media plurality".

18 November James Murdoch warns the Government that if it blocks bid, News Corp could focus future investments overseas, adding that Government must decide whether it wants to risk "jeopardising an £8bn investment in the UK" with a prolonged investigation.

November Cameron attends The Sun military awards reception and dinner awards ceremony; meets Rebekah Brooks and James Murdoch for "social" purposes; meets Rebekah Brooks separate for "social purposes".

9-10 December Andy Coulson gives evidence to the perjury trial of disgraced MSP Tommy Sheridan. Coulson tells the jury he had no knowledge of phone hacking or private investigator Glenn Mulcaire.

10 December A Scotland Yard inquiry has not found any new evidence of criminal activity. The Crown Prosecution Service says no further charges will be brought over the News of the World phone-hacking scandal because witnesses refused to co-operate with police.

15 December Documents lodged at the High Court by lawyers for Sienna Miller allege that NOTW executive Ian Edmondson had knowledge of phone hacking.

21-22 December Vince Cable is stripped of role deciding on takeover after The Telegraph reveals he has "declared war on Murdoch". Brussels clears the deal on competition grounds and responsibility for media competition issues is passed to Culture Secretary Jeremy Hunt. Broadcaster's shares close up 14p – nearly 2 per cent – at 743p.

Thursday 23 December Cameron and his wife attend a dinner at Brooks' Oxfordshire home. Also in attendance are James and Kathryn Murdoch and Jeremy Clarkson and his wife Francie. Clarkson later revealed that within the following days the Camerons and the Brooks also had a picnic.

2011:

5 January NOTW suspends and later sacks Edmondson after claims of phone hacking in 2005-06.

6 January Hunt meets with News Corp to set out the process he proposes to follow in assessing the takeover deal.

7 January Scotland Yard asks the NOTW for any new material it may have in relation to hacking.

21 January Andy Coulson resigns as Cameron's director of communications, saying he has become a distraction.

25 January Hunt says he considers the merger "may operate against the public interest in media plurality" but before referring it to the Competition Commission he says he will take more time to consider News Corp's proposal to protect the independence of Sky News.

26 January Scotland Yard launches Operation Weeting, a new investigation into phone hacking, under Deputy Assistant Commissioner Sue Akers. Police vow it will leave "no stone unturned".

15-16 February Hunt writes to News Corp saying that unless it amends the Sky News proposal to meet the concerns of the regulators, he will refer the merger to the Competition Commission. News Corp replies with a revised Sky News plan.

February Cameron attends NOTW's Children's Champions reception at Downing Street.

1-2 March News Corp to bypass media plurality concerns by spinning off Sky News into separately listed company. Hunt all but nods through takeover in the long term.

March Meets James Harding, of The Times, for a general discussion.

April Coulson invited to Chequers to thank him for his work for Cameron. This month the PM also meets Mohan for a general discussion; James Harding for a general discussion; and he attends the News International summer party and addresses The Times CEO summit.

5 April Edmondson and NOTW chief reporter Neville Thurlbeck are arrested and bailed on suspicion of conspiracy to intercept voicemails.

14 April James Weatherup, another senior NOTW journalist, is arrested and bailed by Weeting.

20 June NI submits recently rediscovered emails relating to the phone-hacking scandal and new allegations that NOTW executives authorised corrupt payments to police.

June PM Attends The Sun's police bravery awards.

1 July Government says it is ready to give clearance to deal. Jeremy Hunt gives opponents a week to raise objections.

6-12 July Ofcom intervention fuels fears deal will not go ahead. NOTW closed down. Ofcom says it has "a duty to be satisfied on an ongoing basis that the holder of a broadcasting licence is 'fit and proper'".

8 July Coulson and former NOTW royal editor Clive Goodman, who was jailed in January 2007 for intercepting voicemail messages of members of the royal household, are arrested and bailed as part of Operation Weeting and Operation Elveden – the Met's investigation into alleged illegal payments to police officers.

14 July News Corp withdraws bid.

14 July Neil Wallis, former NOTW deputy editor, arrested and bailed by Weeting.