The Department of Justice is facing allegations of 'collusion' over its handling of the probe into Hillary Clinton's secret email server.

Donald Trump's campaign questioned the integrity of the investigation after an email on Wikileaks in which one of Clinton's aides said he had had 'DOJ folks inform me' about a hearing.

The actual message was not part of the leak beyond an apparent start to it saying: 'Hey Brian, This was filed tonight.'

The message, from Brian Fallon, her national press spokesman, was sent to Huma Abedin, her right-hand woman, and appears to have been copied to John Podesta, whose emails were leaked.

He's with her: Brian Fallon is frequently seen at Clinton's side, including when she flew home after Sunday night's debate

At the time it was sent there was no criminal investigation into Clinton's server. That was launched two months later by the Department of Justice after a complaint from the Intelligence Community Inspector General and lasted a year.

But the State Department was involved in litigation in the form of suits to make the emails public - in the case of the email, most likely one brought by a Vice News journalist.

The timing of the email suggests that Fallon was informed of the hearing by the Department of Justice because they were acting for the State Department.

The charge of collusion was made by a Trump spokesman - and hours later, Trump himself described a report of the email as 'unbelievable'.

In a statement, Trump's spokesman Jason Miller said: ' Today’s report that Clinton’s campaign was in communication with the Obama Department of Justice on the email investigation shows a level of collusion which calls into question the entire investigation into her private server.

'The Department of Justice must release all communications with the Clinton campaign and her allies as soon as possible in order to definitively prove their investigation was completely above board.'

The impartiality of the investigation has come under heavy fire from Trump and his supporters - and the tone of 'hey Briain' is likely to increase their ire.

Trump cited Attorney-General Loretta Lynch's secret meeting on her plane with Bill Clinton at Sunday's debate and said that if he were to win, he would appoint a special prosecutor to act on Clinton.

The White House declined to comment on the email because it was from 'the stolen email of a private citizen'.

It was the latest in the Wikileaks trove of thousands of emails from John Podesta, Clinton's campaign chair and a longtime family consigliere.

Other emails have disclosed a bitter feud between Chelsea Clinton and Doug Band, her father's one-time closest aide, in which the former First Daughter was called 'a spoiled brat' who 'ran to daddy', while Band was accused of profiting from Bill's name.

Outrage: How Trump reacted to the publication of the Wikileaks email from Brian Fallon

The emails also disclosed how Clinton was given advice on leadership by Sheryl Sandberg of Facebook and told her cmapaign was flatlining among millennials by Howard Schultz of Starbucks.

And they have disclosed how fearful the aides were of the impact of Bernie Sanders, going to the lengths of coming up with pages of 'opposition research on him'.

Clinton's server has dogged her since its existence was first disclosed in March 2015.

The emails revealed a bizarre idea to have her address it shortly afterwards in a 'light-hearted' on-stage discussion with Comedy Central's Larry Wilmore.

The scheme was not proceeded with and the emails reveal that the server issue dogged the Clinton campaign consistently since then.

In July the FBI director cleared Clinton of criminal wrong-doing but called her and her aides 'extremely careless' in their handling of classified information.

The decision has since been criticized by Republicans, criticism which escalated with disclosures that immunity from prosecution was given to Clinton aides, including Cheryl Mills who was her former chief of staff.

She was given immunity so that the FBI would get access to her laptop, and she then also sat in on the agents interviewing the Democratic candidate as her lawyer.