The shadow business secretary reiterated his concerns that many Royal Mail property assets were in prime locations and could be sold, resulting in a windfall for investors and leaving taxpayers' short-changed.
The prospectus highlights sites in London at Mount Pleasant and Nine Elms as being "surplus", with reports saying they are worth between £500 million and £1 billion each, according to Labour.
And Mr Umunna refused to support or condemn potential industrial action by postal workers linked to the controversial sale of the business, which the Government says will be valued at up to £3.3 billion.
He said in an interview that there was a huge coalition of opposition to the Government's plans to sell-off shares in the organisation.
Mr Umunna said: "Now they are pressing on with it, what they need to illustrate is they are going to get the best value for the taxpayer and increasingly what this is looking like is a massive bonanza for city speculators and a huge short-changing of the taxpayer at the same time. It is not too late to pull the plug.
"Until you've dished out the shares, until you've allocated them it is free for the Government to say we have taken a step back and decided not to go ahead with it."