Detectives investigating the cash-for-honours affair have uncovered a "hidden" computer network in Number 10 from which crucial emails appeared to have been deleted, according to claims.

Downing Street issued a swift denial to the allegations made by ITV News, which claimed the existence of a second IT system in the building was revealed to police by a witness over recent weeks.

The information reportedly led to the arrest of one of Tony Blair's closest aides, director of Government relations Ruth Turner, last week.

ITV also alleged that officers discovered emails last autumn which were apparently exchanged between Labour fundraiser Lord Levy, Ms Turner and Mr Blair's chief of staff Jonathan Powell.

They were said to be "indiscreet" and referred directly to "K's and P's" being offered to those who have donated money. The suggestion was that K could stand for knighthood and P for peerage.

A Downing Street spokesman said: "This story is untrue. There is no second computer system in No 10. There are no emails of the type that seem to be described. The police have had full access to the system and full co-operation. "The police have not put any of this to us."

Some emails on the second computer network appeared to have been deleted after the police inquiry was launched, ITV said.

Detectives reportedly now expect the inquiry to run until the spring, but charges remain unlikely because of the difficulties proving "criminal intent".

The arrest of Ms Turner - one of the Prime Minister's closest aides - sent shockwaves through Westminster last Friday. The police decision to detain Ms Turner at her London home in a dawn raid led to accusations from Government ministers and Labour peers that they were being heavy-handed and "theatrical".

There were also indications that the move may have been due to new information suddenly coming to light rather than designed to "shake up" suspects.