By Nick Assinder BBC News Online political correspondent

Another day - and another demand for the sacking of Transport Secretary Stephen Byers.

The beleaguered minister is, this time, accused of misleading the House of Commons over the "spingate" affair.

And the evidence certainly seems stacked against him.

Sixmith did not quit

Downing Street has confirmed that it was clear there was an "incorrect understanding" over whether civil servant Martin Sixmith had resigned or not in February long before Mr Byers appeared before MPs on 26 February.

In other words, the minister knew there was a doubt over the resignation before he made his statement entitled "the resignation of Martin Sixmith".

And the statement issued from the Department of Transport last Tuesday now confirms that Mr Sixmith had not resigned at the time his resignation was announced, or at the time Mr Byers made his statement.

Full confidence

Yet the prime minister told MPs that Mr Byers did not mislead the House either deliberately or inadvertently.

And he has let it be known he still has "full confidence" in his minister.

Byers' apology demanded

The issue now is over demands for the transport secretary to follow established practice and return to the Commons to explain how he misled MPs, and to apologise.

So far, those demands are being resisted with the persistent claim he did not mislead MPs.

But it is difficult to see how that line can hold indefinitely.

Did not mislead

Mr Byers' original statement on 26 February was made because, during a TV interview, he had given a "misleading impression" of his role in the resignation - or non-resignation as it now turns out.

He went before MPs to apologise for that error. And, with the prime minister's support, held onto his job despite the clamour for his head.

Blair's full confidence

But now, MPs are being told there is no need for a similar statement and apology because he did not mislead them.

Put bluntly, the opposition and others, do not believe this. And Iain Duncan Smith made that plain during some heated Question Time exchanges with the prime minister - but to no avail.

The opposition points out that Mr Byers' words on 26 February were unambiguous.

He told MPs: "Martin Sixmith offered his resignation, which was accepted, on 15 February."

Deaf ears

Yet it has now been confirmed that, as Mr Sixmith always claimed, he had not resigned.

That looks like a direct case of MPs having been misled, for whatever reason.

The calls for Mr Byers' head or an apology will continue, but there are no signs whatsoever that the prime minister is about to sack him or force him to appear in the Commons.

And most MPs seem resigned to the fact that their demands will fall on deaf ears.

What is certain, however, is that this issue will not go away and, sooner or later, Mr Byers will have to face the Commons, even if only in his regular departmental question time.

There are also few who now believe the transport secretary will survive a summer cabinet reshuffle.