Second former senior News of the World journalist to also be arrested after leaks from NI force police to speed up plans

Andy Coulson has been told by police that he will be arrested on Friday morning over suspicions that he knew about, or had direct involvement in, the hacking of mobile phones during his editorship of the News of the World.

The Guardian understands that a second arrest is also to be made in the next few days of a former senior journalist at the paper.

Leaks from News International forced police to speed up their plans to arrest the two key suspects in the explosive phone-hacking scandal.

The Guardian knows the identity of the second suspect but is withholding the name to avoid prejudicing the police investigation.

Coulson, who resigned as David Cameron's director of communications in January, was contacted on Thursday by detectives and asked to present himself at a police station in central London on Friday, where he will be told that he will be formally questioned under suspicion of involvement in hacking.

After being questioned by detectives from Operation Weeting – a process that could take several hours – the former rising star of News International is likely to be released on bail conditions that include appearing at court at a later date along with his three former colleagues who have already been arrested: Ian Edmondson, Neville Thurlbeck and James Weatherup.

The arrest will be embarrassing for Cameron, who consistently defended his decision to hire the controversial former journalist amid mounting evidence of his involvement in the hacking scandal.

Coulson was editor of the News of the World between 2003 and 2007. A close friend and deputy of the News International chief executive Rebekah Brooks when she edited the paper, Coulson resigned a few weeks before the paper's royal correspondent, Clive Goodman, and private investigator Glenn Mulcaire were jailed after admitting intercepting messages on royal aides' phones.

In July of that year, he became the head of the Conservative party's media operation and then communications chief for the prime minister after the formation of the coalition government in May 2010.

He has always strenuously denied any knowledge of the illegal telephone hacking that is at the heart of the scandal rocking the Murdoch empire.

When he resigned in January from his Downing Street role, he insisted he had done so because persistent allegations that he must have known that his reporters had been hacking into voice messages had made it impossible for him to continue.

Coulson is one of three News of the World journalists whose evidence to the trial of Scottish MSP Tommy Sheridan is being examined after doubts were cast on his claim that he was unaware of any wrongdoing by News of the World journalists.Evidence leading to the two imminent arrests has come from a cache of emails recently uncovered during News International's internal investigation into phone hacking.

The arrests had been planned to take place before 8 August, when Operation Weeting had agreed to pass all the relevant material in their possession to lawyers acting in the civil cases against News International for victims of phone hacking – thereby giving suspects the opportunity to discover what evidence the police hold against them.

The Guardian understands News International had promised police they would not make public the existence of evidence identifying Coulson and the other journalist, but that detectives began to fear the information would be leaked, after reports appeared suggesting that Coulson approved payments to police officers.