Scotland Yard bosses have admitted a goal to recruit 10,000 special constables to help police the 2012 Olympics is unattainable.
Senior officers were under pressure to treble the ranks of volunteers to help secure the sporting extravaganza.
But there have been concerns at the number of officers leaving their posts, as well as their professionalism and the cost of training them.
As a result, top Met officers have agreed to set their sights on signing up 6,667 officers by March 2012.
And they have warned the goal of amassing a 10,000-strong workforce may not even be reached three years later.
Commissioner Sir Paul Stephenson has been told the higher target, set by London Mayor Boris Johnson, was an "aspiration" and is "probably unachievable".
He has agreed to draw £1.4 million from reserves to pay for the recruitment of specials after the Home Office cut funding.
The decision will be seen as a blow to Government plans for a revolution in volunteering across many parts of the public sector.
Ministers have said they want to "unlock the potential" of specials, and want to consider creating ranks of police reservists based on the Armed Forces model.
There are around 3,350 specials working in London on patrol and in some specialist units. They carry warrant cards and have the same powers as full-time colleagues.