Four hundred and forty police officers are being seconded to help tackle illegal immigration in the UK.
The UK's immigration staff would have greater powers of arrest
They will be among 800 new immigration staff - a 25% increase in total - unveiled by Home Secretary John Reid.
The plans will allow Britons involved in people-smuggling to be arrested, firms to face larger fines, and the public encouraged to report suspects.
But the Tories said police were "badly needed" on the beat, while the Lib Dems want "new, not redeployed" staff.
The 400 police constables and 40 sergeants moving across to the Immigration and Nationality Directorate will be joined by 360 newly-recruited immigration officers.
"We are very clear that we do not want this to have an impact on police officers' frontline work," a Home Office spokeswoman insisted.
"There won't be any police officers taken off the front line," she said.
But shadow home secretary David Davis criticised the government's record on the subject.
"Over the last nine years, we have actually seen immigration officers instructed not to arrest illegal immigrants, merely to meet the prime minister's artificial targets on removing failed asylum-seekers."
"People-trafficking and all its associated evils is one of the fastest-growing and most vicious crimes, yet the government's policy so far has been one of neglect."
And for the Liberal Democrats, home affairs spokesman Nick Clegg said that "a truly integrated border force" needed more than to "shift police officers away from their current duties".
"The government itself has admitted it would cost £104m to secure every port of entry into this country," he said.
"Why then has John Reid failed to announce any new funding while tens of thousands of pounds a day are wasted on identity cards?"
Under the plans, members of the public will - for the first time - be able to report illegal workers and illegal immigrants using the free Crimestoppers telephone line from 1 January.
The government has already said it was doubling the budget for deportations to nearly £300m, and a bill tackling deportations was included in last week's Queen's Speech.
And ministers intend to create 650 extra detention spaces for illegal immigrants by the end of 2007.
The measures are seen as a response to criticism from political opponents about a perceived failure to deport enough of those identified as being in the UK illegally.
There were nearly 5,000 deportations in the last three-month period for which figures were available - a record - but it estimated that 500,000 people remain in the UK illegally.
The idea of penalising businesses did not find favour with the Immigration Advisory Service, which said firms would become "reluctant to recruit anybody".
"I'm fearful that's going to lead to discrimination against anybody who looks as though they might be a foreigner," chief executive Keith Best told BBC Radio Five Live.