The prosecution's star witness in the case against media tycoon Conrad Black has been called a liar by the defence - and admitted the accusation is true.

The drama unfolded in court this week as Black's partner in the publishing business, David Radler, took the witness stand.

'I wasn't totally truthful,' Radler admitted.

'Not totally truthful – is that what you call it?' Black's defence lawyer Edward Greenspan demanded.

Radler finally cracked: 'Call it lying.'

Greenspan accused Radler of serious discrepancies in his testimony and in the statements he gave to a special committee in 2003.

He said Radler never told the committee - which was investigating shareholder concerns - that his boss, Conrad Black, had a plan to funnel money away from Hollinger International. He said he initially lied about his role in a plan to pocket 'non-compete' fees because he was afraid of the consequences.

Greenspan said Radler is doing the same thing in the courtroom and is lying to keep a deal with prosecutors that will see him serve a light sentence.

When asked how many - of 24 federal investigators he met during his guilty plea – he had lied to, Radler said: 'I'm going to say a fair number of them'.

The critical admission of Radler is set to seriously hurt the prosecution's case, as he was the first government witness to testify in the trial in which Black is accused of committing fraud against shareholders of Hollinger International Newspaper.

Payments for assets of the company were allegedly deflected to a Canadian company managed by Black.

The prosecution alleges that Radler and Black robbed Hollinger of more than $60m by concealing the payments.

Radler pleaded guilty – for which he is expected to receive a jail sentence of 29 months – and then agreed to testify against his former partners in the fraud trial.

Further reading:

Star witness says Conrad Black knew of all deals

Conrad Black Endures the Indignities of a Trial

Conrad Black's lawyer rips into key witness