The BBC's Political Editor Andrew Marr considers where the latest claims leave the saga.

What is the latest situation?

The latest is a sensational story in The Sun which appears to have some 24 hours worth of Peter Foster's mobile telephone conversations.

The tapes and the transcripts are allegedly conversations he had with his mother.

It seems as if they were being recorded by somebody and sent to The Sun.

What is the impact of the latest revelations?

The effect is to paint Mr Foster as a sleaze-bag who is in it for the money and hostile to Cherie Blair.

It comes on a day when Number 10 most needs to grind Mr Foster's already pretty dirty reputation into the mud.

The question now is who, therefore, obtained the private telephone conversations and passed them to The Sun.

Where does the balance lie between Number 10 and its critics?

Somebody is lying, and lying repeatedly.

Number 10's case is that the lies come from Peter Foster and the people around him and that there is a concerted and vicious attempt to entangle the Blairs into his extradition case and drag them down.

On the other side you have had the stories in other papers such as the Daily Mail and The Scotsman that suggest that the Number 10 press office and the people around there have been lying repeatedly.

In the end one side or the other is going to completely lose its reputation.

This is about as filthy, multi-layered and aggressive a battle over reputations as you will ever see in politics.

Is Tony Blair right to challenge his accusers to take evidence of any wrong-doing to the authorities?

He has to do that.

He has to front up the critics and demand the evidence and if necessary take it all the way to court.

This is not simply now about who sent what fax to whom and at what time or who read what document.

It is about whether or not you can trust the word of the prime minister of the United Kingdom and those around him.

It is about the authority credibility of the prime minister - it doesn't come bigger than that.

It may have started as a very small looking story but it does not look that way now.