The row claimed the scalp of Immigration Minister Beverley Hughes after she admitted "unwittingly" misleading the public.

Conservative leader Michael Howard branded the government's immigration policy "a complete shambles".

And he accused Labour ministers of "sitting on their hands" on the issue.

'Scandal'

Ms Hughes had insisted she had not known about Romanians and Bulgarians using forged documents to enter the UK.

Who knew about the disasters in Romania and Bulgaria? David Davis Hughes: I didn't try to mislead Beverley Hughes: Letters

But later it emerged a Labour whip had told her about it a year ago. Mr Blunkett, her boss, described Thursday as the worst day of his political career.

The Tories are putting pressure on the home secretary to say exactly when he knew about reports of the scam.

Mr Howard told Welsh Tories at their Llandudno conference it was a "scandal" that officials have had to work "under an intolerable burden" and that "sham applications - from one-legged roof tilers, fake electricians and bogus builders - have had to be rubber-stamped."

Earlier, shadow home secretary David Davis repeated his call for an independent inquiry into the events leading up to Ms Hughes' resignation during an interview on BBC Radio 4's Today programme.

I have decided that I cannot in conscience continue to serve Beverley Hughes Resignation statement

He said a number of questions still remained unanswered.

"Who knew about the disasters in Romania and Bulgaria and anywhere else we find them because we don't know that's the end?" he asked.

"When did Beverley Hughes know, when did David Blunkett know - at what point was he involved in this process?"

Immigration card?

Something was "systemically wrong" in the current immigration system, Mr Davis added.

But former Labour home office minister John Denham told Today: "The Tories are undoubtedly playing the immigration issue much more overtly - for the votes they think are in it - than has been the case under the last two Tory leaders."

The row started after Mr Davis received an e-mail from British diplomat to Romania James Cameron who said the Home Office were approving visa claims from eastern Europe despite warnings they were backed by forged documents.

Ms Hughes said she had not set out to "intentionally mislead anyone", but she could not "in conscience continue to serve as immigration minister".

WHO KNEW WHAT WHEN March/April 2003: Bob Ainsworth raises concerns with Beverley Hughes Monday night: Ms Hughes tells Newsnight she knew nothing about scam claims until they were raised by the Tories Tuesday evening: Mr Ainsworth reminds Ms Hughes of the letters when he bumps into her in Parliament. Tuesday dinner: Ms Hughes discusses them with her boss, David Blunkett Wednesday: Ms Hughes sees the full papers. Tony Blair told about the letters before prime minister's questions Source: Downing Street Reaction from immigration groups Whistleblower claims vindication Civil servants' roadshow en route

The prime minister has replaced Ms Hughes with Work and Pensions Minister Des Browne.

Mr Blair denied reports he would be taking personal charge of immigration but said he would take "a close interest" in the issue.

"It's pretty obvious to me that something was seriously wrong," he told his monthly news conference.

The question of whether fraudulent claims were deliberately approved - or whether officials mistakenly believed the applications had to be allowed - is still being investigated.

But the prime minister dismissed suggestions that Home Secretary Mr Blunkett should also resign.

Ms Hughes had told BBC Two's Newsnight on Monday that Mr Cameron's claims were the first she knew of the alleged "scam".

He urged people not to assume the same problems were happening across the whole immigration system, stressing that the vast majority of immigrants were lawful and made a great contribution to Britain.