Ministers have been accused of "deliberately deceiving" the British public over a secret plan to increase immigration.
Tory shadow home secretary Chris Grayling said ministers had waved through hundreds of thousands of new arrivals without proper checks.
The Government then engaged in a "cover-up" to hide revealing documents and broke information access laws, he claimed.
Mr Grayling launched his attack in an emergency Commons statement following claims that 337,000 visas had been fast-tracked. A national newspaper alleged officials had been encouraged to take risks by not carrying out full checks on applicants. It claimed the Home Office "illegally withheld" internal emails relating to the scandal.
But immigration minister Phil Woolas said the allegations had been fully investigated when they first emerged in 2004. And he said the Government had complied with the Information Commissioner's demand that the emails be released and published more than six months ago.
Former immigration minister Beverley Hughes, who resigned when details of the scandal first emerged, had "acted entirely honourably", Mr Woolas said. The correspondence from March 2003 appeared to suggest she knew of the policy and approved it. An email from a senior official read: "We are still in a situation where some risks have to be taken, and staff should feel that if they are encouraged to take risks they will be supported when something does go wrong."
A reply sent by Ms Hughes's office three days later said that the minister had "seen and noted" the submission. Mr Grayling told the Commons the story added further weight to claims by a former speechwriter, Andrew Neather, that Labour had tried to deliberately engineer mass migration.
He said: "More and more evidence is now emerging to suggest that the Government broke Freedom of Information laws and tried to cover up a deliberate change of policy designed to encourage much higher levels of immigration - very probably for party political purposes. It is now clear that this is a Government that has set out to deliberately deceive the British people and a Government that has proved utterly incapable of telling the truth about its policies on immigration."
As the row blew up in Parliament Home Secretary Alan Johnson called for a full debate on immigration. He said a discussion involving the "moderate majority" would help tackle the threat from the BNP. Mr Johnson said: "Let's not have a debate as if, on the one hand, there's an argument for an open door policy and on the other hand the argument is to close the door completely. No-one in mainstream politics is arguing for any of those alternatives."
