The government must clear up "confusion" over what it means when promising to protect "front-line" policing staff from cuts, MPs say.
The Commons home affairs committee said there would be "significantly" fewer officers in England and Wales due to budget reductions of 20% by 2014-15.
Disagreement over which officers were judged to be front-line made it harder to judge the impact of cuts, it added.
The government said it was working to reach a "better definition".
The home affairs committee's report looks at the impact on police forces of financial settlements proposed from 2011 to 2015.
It says, while there is "uncertainty" about precise figures, fewer a reduction in the number of officers, community support officers and police staff is expected.
'Thin blue line'
The committee's chairman, Labour MP Keith Vaz, told the BBC: "The fact is we are going to have significantly fewer police officers than we have ever had before.
"Of course individual chief officers have said that front-line policing will be affected, but the government has made it very clear that they don't believe it's going to be affected.
"So what we need is a definition of what is the front line. What is the thin blue line? What is the middle office and what are the back offices? And how are fewer police officers going to provide a better service?
"What is absolutely clear to the committee is that policing will never be the same again and therefore we are going to look again at the way in which the whole landscape of policing is to operate in the future."
Mr Vaz said police would have to do "different activities than they have done before" because there would be fewer of them.
"I think the day when you could pick up a phone and get a police officer to come round and take a statement from you if your house has been burgled has now gone," he added.
The committee says, with fewer posts, it is crucial that staff's time is managed effectively and that "unnecessary bureaucracy" must be reduced and contracts to buy services and equipment must be better co-ordinated.
The committee also raises concerns that the biggest savings are being made in 2012-3 - at a time when police authorities are being replaced with police and crime commissioners, and when there will be additional pressure on forces nationwide due to the 2012 Olympics.
It urges the government to acknowledge the "risks involved in this transition".
Policing Minister Nick Herbert said there was additional funding for Olympic security and the costs of new police and crime commissioners would be no more than the current system. A "transition board" had been set up to manage the change.
He added: "We believe that by controlling costs, cutting bureaucracy, making savings in force back and middle offices and improving productivity, the police service for the public can be maintained and improved even as funding is reduced."
But he said: "We agree that a better definition of front-line policing would be helpful and Her Majesty's Inspectorate of Constabulary are discussing this with the police service."
He also welcomed the committee's comment that "there may be scope for reducing headcount in the police".
For Labour, shadow home secretary Yvette Cooper said the government should "think again on its deeply destructive cuts to the police".
She said "front-loading" the cuts in the first two years meant it was harder to avoid hitting front-line services.
"The home secretary and the prime minister have consistently claimed they will protect the front line. But astonishingly, as the select committee points out, ministers can't even explain what front line means. This is a cavalier approach to public safety and a breach of trust."