Conservative Party leader Michael Howard said on Monday that people who previously would have been detained were now being let into the UK and told to report back after 48 hours.
The Home Office replied there had been no policy change and anyone thought to pose a security risk was detained.
Both the Tories and Liberal Democrats want a separate border police. Labour says it is tightening immigration controls.
In response to allegations made during the Conservatives' manifesto launch, a Home Office spokesman said: "There has been no change in immigration policy on granting temporary admission.
"The circumstances of each individual case are carefully considered by an immigration officer before temporary admission is granted.
"Passengers who arrive at any airport are never granted temporary admission if it is believed that they are a security risk or a threat to society."
The Tories first made the claims two weeks ago, based on an article in the News of the World. Party chairman Liam Fox said the chief constable of Avon and Somerset police had personally told him the policy had changed.
The Conservatives have made immigration a key election issue, pledging annual immigration quotas, bonds for temporary workers and offshore processing of asylum claims.
On Monday, Howard said immigration officers at some airports had been told to allow people with suspect papers to enter the UK "in the hope they will come back and report at the end of those 48 hours".
"These are people in relation to whom there is a suspicion that they are coming into the country, that they are tricking their way into the country, possibly for nefarious purposes," he said."