The Andrew Mitchell “plebgate” affair deepened today after it emerged police have arrested and questioned a second man.
The 23-year-old, who is not a police officer or member of police staff, was arrested last night on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around December 14, Scotland Yard said.
He was questioned at a London police station and released on bail this morning to return in January.
It remains unclear exactly what the arrest relates to, but the date of the alleged offence - December 14 - was one day after police received fresh information on "plebgate" and a day before a police officer was arrested on suspicion of misconduct in public office.
Yesterday, police widened the investigation amid signs of strain between senior Conservatives and the police over the treatment of Mr Mitchell, as claims were made that officers conspired to fabricate evidence against the former chief whip, which ultimately led to his resignation in late October.
Around 30 officers are working on the inquiry, which is being supervised by the Independent Police Complaints Commission (IPCC).
Prime Minister David Cameron expressed concern that an officer tried to "blacken the name" of Mr Mitchell, amid mounting questions over the initial account of his row with Downing Street police who refused to let him ride his bicycle through the main gates.
The officer is said to have emailed his local MP, Conservative John Randall, posing as a member of the public and accusing Mr Mitchell of calling police "plebs".
The account in the email, written the day before The Sun first broke news of the row on September 21, was very similar to that in the police log, which was later leaked to the Daily Telegraph.
Scotland Yard yesterday said its investigation could look at the possibility of a conspiracy.
"The allegation that a serving police officer fabricated evidence is extremely serious," it said in a statement.
"It goes to the very heart of the public's trust in the police service."
London Mayor Boris Johnson today described the allegation that a police officer wrongly claimed to be an eyewitness as "very serious indeed".
Speaking to Nick Ferrari on LBC 97.3 radio, he said: "Clearly, there's a serious question to answer by the member of the diplomatic protection squad, the officer concerned, who is alleged to have sent a fallacious email.
"That's got to be sorted out.
"It's not just serious because it affected the career of a Tory Cabinet minister, or whatever - it's serious because everybody in London has a right to expect the police to be fair and scrupulous in their testimony about what may or may not have happened."
Attention also turned to the Police Federation as former home secretary Kenneth Baker said the body was in "real trouble" over the affair.
Lord Baker told BBC2's Daily Politics: "The police must be above politics in our country.
"The Police Federation decided to become a lobbying, an extremely aggressive lobbying body, determined to get a minister - a Cabinet minister - because they were very opposed to what the Home Secretary is doing looking into their pay, pensions and early retirement."
In the email to Mr Randall, which was passed to Downing Street on September 25 and published by Channel 4 News on Tuesday, the writer says he was walking past Downing Street with his nephew when he recognised Mr Mitchell.
In the message, which is littered with mistakes, he said: "Imagine to our horror when we heard MR MITCHELL shout very loudly at the police officers guarding YOU (expletive blacked out) PLEBES!!" and "YOU THINK YOU RUN THE (expletive blacked out) COUNTRY" and just continued to shout obscenities at the poor police officers."
The email sparked the series of events that led to Mr Mitchell's resignation.
Mr Randall, his deputy in the Government whip's office, is believed to have suggested he would quit unless his boss left his post.
Further questions about the account of Mr Mitchell's row with police emerged in relation to CCTV footage of the incident.
Although there is no sound, the MP can be seen with his bicycle talking to three officers by the main gate for around 20 seconds, then wheeling it to the side gate and leaving.
It appears to show few members of the public passing by - apparently contradicting police records.
Mr Mitchell has demanded a full investigation into the police account of events, insisting the email was key to the loss of his job.
Downing Street said it received two emails via Mr Randall, but after comparing their claims with CCTV, Sir Jeremy decided they were not "reliable".
Sir Jeremy did not look at the Downing Street police log as part of his review, a spokesman said, adding that the Prime Minister's view was that reasonable inquiries were made.
Chairman of the Police Federation of England and Wales Paul McKeever acknowledged concerns that his organisation had "stoked up" media attention in Mr Mitchell, and said he would apologise to the MP if the current investigation showed he had been wrongly accused.
Mr McKeever stressed that the national federation had held back from calling for the then chief whip's resignation, though local federations in some of the 43 police areas of England and Wales - which each operate independently - went much further.
He suggested that there may be changes to the organisation's structure to allow greater central control over the messages sent out by the Federation.
Mr McKeever told BBC Radio 4's PM that he was personally "taken aback and shocked" by Mr Mitchell's reported comments, which came immediately after the murder of two womanpolice officers in Manchester.
He admitted he "came pretty close" to demanding the minister's head in the days immediately after the row blew up, but said that within days the federation was taking the line on a national level that it was not calling for his resignation.
But he said that it was for each of the 43 local federations "to reflect the views of their members", adding: "I can't criticise the local federations for the line they took.
"It's for them to account for their actions."
Mr McKeever said: "I understand those who say the Federation stoked up some of the media attention in relation to Andrew Mitchell."
And he added: "I think we can all say we could have done things better.
"The federation has to look at how we can get some measure of control to ensure that there is perhaps a better way of doing business in the future.
"That's something we are going to have to review in the New Year."
Asked whether he felt local federations had been unfair to Mr Mitchell by wearing "Pc Pleb" T-shirts and calling for him to be sacked, Mr McKeever said: "We have to wait for this investigation to take place to find out exactly what has happened - and we are supporting that investigation absolutely.
"I am going to wait to see what happens in relation to the investigation.
"If he has been done a calumny in relation to what happened, I will be one of the first in the queue to apologise."
Mr McKeever said it was important to maintain public trust in the police.
He told PM: "It causes us concern that there are arrests taking place now, but that is clearly part of an investigation and that is a judgment made by those who are carrying out that investigation.
"It is vitally important that the public do have trust in us, so it is important that this investigation gets to the root of whatever has happened here.
"I can understand when people say there is a danger that trust might be eroded in the British police service as a result of what's happened.
"But we have to put this in context.
"There are 135,000 very fine men and women across the country doing their utmost on behalf of their communities.
"At the moment, we are talking about one or two officers.
"Clearly, we have to wait for the investigation to take place to find out definitively what happened."
Meanwhile, the president of the Police Superintendents' Association, Derek Barnett, said that police felt "bruised" by recent events and action was needed to bring their relations with Government "in from the cold".
Chief Superintendent Barnett told Radio 4's World at One: "The police service at the moment is feeling quite bruised.
"It is important for the conduct of public life that there is a good relationship between the police service and Government.
"I think it is time - and I have said this before - (to do something) about bringing the police service in from the cold and really establishing good relations with the Government.
"So, early in the New Year, I will be meeting the police minister and this will be top of our agenda."
Scotland Yard said later the 23-year-old man arrested as part of the investigation was detained at his home address in north London.
A force spokesman said: "The man was held on suspicion of intentionally encouraging or assisting the commission of an indictable offence - and that indictable offence was misconduct in a public office."
The Met said officers working on the investigation - known as Operation Alice - have searched the homes of both men arrested.