How did this dispute start?

Immigration is always a hot political potato and got even hotter on 7 March when whistleblower Steve Moxon claimed key checks were being waived at the Sheffield office of the Immigration and Nationality Directorate.

Mr Moxon told newspapers the change affected migrants from the eight eastern European countries due to join the EU in May. He claimed the idea was to rush through as many applications as possible now to make the numbers coming to Britain less dramatic when the EU expanded.

And was he right?

The Home Office suspended Mr Moxon and then admitted some checks had been waived, but said it was local officials trying to clear a backlog, rather than ministers seeking to massage the figures. An internal inquiry by senior official Ken Sutton backed this position.

The Home Office said the change only affected checks under the European Community Association Agreement (ECAA) scheme - for prospective business people from the east European countries joining the EU on 1 May.

So why wasn't that the end of the row?

This weekend saw new allegations emerge as the Sunday Times printed a leaked memo from senior managers at the Immigration and Nationality Directorate's offices in Croydon, suggesting a more widespread waiving of checks on those seeking permission to live and/or work in the UK.

It said the backlog of applications on migrants who had been in Britain for more than three months should be cleared "as quickly as possible" and said Ms Hughes had approved this policy change.

The Tories seized on the report as evidence that fast-tracking extended way beyond Sheffield and that ministers knew about it.

Did ministers put their hands up after that memo, then?

They admitted there was fast-tracking but denied it was secret and said it was in line with similar measures taken from 1988 onwards under successive home secretaries, including Tory leader Michael Howard.

Home Secretary David Blunkett said the change covered only people who were already in Britain and was in response to cases like the US-born grandmother threatened with deportation after nearly 54 years living in the UK.

And he gave just about as robust a defence as possible of a junior minister, telling the "right-wing press" to forget it if they thought they were going to get the "scalp" of Ms Hughes.

But now there's another whistleblower?

Yes and his name is James Cameron, a senior British diplomat in Bucharest, Romania.

It turns out he e-mailed Tory shadow home secretary David Davis on 8 March, after hearing about Mr Moxon's initial claims, saying the concerns raised in Sheffield were just the "tip of the iceberg".

What are his specific allegations?

He says the waived checks under the European Community Association Agreement scheme were also being applied to Romania and Bulgaria, which are not due to join the EU until at least 2007.

He said that when British officials in Romania told the Sheffield Immigration and Nationality Directorate that applications were being made using forged documents "their letters were ignored".

The claims are much more serious - something Mr Blunkett admits - because they would mean people were knowingly ignoring health warnings on what the Tories say could be hundreds of thousands of immigration claims.

What has happened to Mr Cameron?

He was suspended after being interviewed by the Foreign Office last week.

Critics are questioning how, if he had already been interviewed, Ms Hughes could say she did not know what his concerns were until Mr Davis released them on Monday.

Ministers are also accusing the Tories of playing politics by sitting on the e-mail for three weeks. Mr Davis says he has had to wait until now to be able to back up the claims.