I have picked this story up from the excellent EU Referendum website and I thought I would pass on the bare bones of the story. It starts with a question during Prime Minister's question time where Ms Harman was deputising for Mr. Brown:
Q3.  Ann Winterton (Congleton) (Con): Will the deputy Prime Minister confirm that the real reason for part-privatising Royal Mail stems directly from European Union postal legislation, which forced Royal Mail to divest itself of its most profitable business, thereby handingit over lock, stock and barrel to European competitors? What sense is there in that?
Ms Harman: The real reason, and the basis on which we are bringing forward the Postal Services Bill, is the analysis in the Hooper report, which we commissioned as long ago as December 2007. It made it clear that we need to take action to put Royal Mail, which, as the Prime Minister has said, is part of the fabric of our society, on a firm footing for the future. That means that we have to ensure that the pension liabilities are met. We have to ensure that the unfair regulation is tackled. We have to ensure that there is legislative underpinning of universal postal services, and also that we get into the organisation—so that, as well as meeting its pension liabilities, it can also modernise—considerable public capital investment but also private capital investment. When we bring forward that Bill to support the future of Royal Mail, I hope that the hon. Lady and all other hon. Members will support it.
If one looks at the Hooper report then we find that the reason we need to 'put Royal Mail ... on a firm footing for the future' is stated in Box 12 on page 81 where it says 'European directives require that all European postal markets must be fully open to competition by 2012.'
So why couldn't Ms Harman say so? Now you know why those back-benchers who are against this bill will quietly disappear when it comes to the vote. They, like you and me, have no choice in the matter.