Ms Blears' spokesman said she would be "extra careful" from now on
Hazel Blears almost gave the media a sneak preview of plans to reform MPs' expenses when she was photographed holding a copy in Downing Street.
The communities secretary inadvertently allowed the press corps a glance at a sheet outlining Gordon Brown's proposal to end second homes allowances.
But, luckily for the minister, the error was not noticed until after the PM announced his plans a little later.
Ms Blears' office said she was feeling "understandably sheepish".
Several ministers have had problems with documents being mislaid or mistakenly revealed in recent months.
These have been seen merely as embarrassments, rather than resignation issues.
However, two weeks ago Metropolitan Assistant Commissioner Bob Quick stood down after he was photographed standing in Downing Street with secret documents about forthcoming anti-terror raids in full view.
As Ms Blears left Tuesday's cabinet meeting, press members opposite 10 Downing Street took several pictures of her.
In them, the smiling communities secretary is holding a file in her left hand.
The wording on the file was clearly captured by cameras
Outside the folder - in full view - is a page detailing Mr Brown's plans to abolish the MPs' controversial second homes allowance - worth up to £24,000 a year - in favour of flat-rate daily expenses for attending Parliament.
By the time Ms Blears' mistake was noticed, the prime minister had laid out his plans in a video message on the Downing Street website.
Ms Blears, Labour MP for Salford, has apologised to Mr Brown, but it is not the first time she has been caught out in this way.
Last May she was photographed leaving Downing Street holding an e-mail printout on the possibility of the prime minister taking part in an Apprentice-style reality TV programme, to be called Junior PM.
The same day, the then housing minister Caroline Flint walked past photographers carrying a see-through file as she arrived to brief the Cabinet on problems in the housing market.
And Culture Secretary Andy Burnham has apologised for leaving "restricted" documents on a train on Monday.
Following Ms Blears' unintended display of Mr Brown's expenses reform plans, a Department for Communities and Local Government spokesman said: "The secretary of state is always very careful when handling all restricted or secure papers.
"The papers that were snapped today were not of that kind and related to an announcement that was about to be made public.
"Nonetheless in this instance Ms Blears is feeling understandably sheepish, she has apologised to the PM and takes it as a lesson to be extra careful in future."