Who do you believe - the police or the politicians?

Supporters of Andrew Mitchell say he is the victim of a serious injustice

What began as an argument about what a minister did or did not say one night at the gates of Downing Street has now turned into a full-scale battle about who can be trusted to tell the truth.

A battle which now involves not just a former government chief whip with a deep grievance, not just his high profile supporters in all the main political parties, but the home secretary herself.

All are now calling on the chief constable of West Mercia to reverse his decision not to discipline an officer on the grounds that he showed "no deliberate intention to lie".

What brings those politicians together is the anger at what they see as a conspiracy orchestrated by the coppers' union - the Police Federation - which has for many years fought home secretaries who sought to alter their pay or conditions.

That and a fury that if police officers can - in their eyes - fit up a minister of the Crown, how can the public believe that they will be fairly treated?

Ever since the CCTV of that night in Downing Street emerged, serious questions have been raised about the truth of the police's version of events.

(A decision is expected soon about whether those who claimed to have heard the word pleb will themselves face prosecution)

Today was merely act one in a drama with a long way to run.