The dramatic confession was written by economist dr. Calvert Jump made in a new book edited by Mr. McDonnell.
Dr. Jump wrote that foreign shareholders in nationalized companies during nationalization have "legally claim to compensation" under contracts signed by the United Kingdom.
He explains, "This is not a trivial problem in the UK.
"As you know, 54 percent of ordinary shares in UK public companies listed on the London Stock Exchange are held by foreigners."
The Labor Party has announced it would nationalize Royal Mail, as well as British rail and water companies, should they come to power.
Dr. However, Jump conceded that compensating foreign shareholders would be a "great difficulty".
In the case of Royal Mail, he predicted that nationalization would cost "compensation to shareholders of hundreds of millions of pounds" compared to the privatization of the company in 2013.
The approval of Dr. med. Jump directly contradicts Mr McDonnell's claim that the nationalization plans of Labor do not cost the public.
Last week, he told the BBC that nationalizing the BBC would "not cost the taxpayer anything, because what we do is making an asset that will give us an income."
He added that the price to which the companies are being brought is "determined by Parliament".
Conservative MP Chris Philp said: "Labour's own advisers admit their plans cost hundreds of millions more, which mocks John McDonnell's farcical claims."
Regarding the comments of Dr. Jump, a Labor spokesman said, "The authors of this book participated in their personal trait.
"Dr. Jump is not a legal advisor to the Labor Party."
It comes when MEP Lloyd Russell-Moyle called for privately owned homes to be seized by the state.
At a Labor meeting, he said, "Let's not just talk about council houses. Let's get those damn private houses back into our hands.
"We need to develop a system that over time takes ownership of private property and puts it in the hands of the public."
Mr. Russell-Moyles proposal of the municipalities should receive the right to purchase houses for sale in front of private buyers.
Tony's deputy Grant Shapps attacked the plan and said, "The truth is out, the cuddly Corbyn wants to nationalize your home."