Labour voters should not be "conned" into believing quitting the European Union will solve their concerns about immigration, Peter Mandelson has warned.
Leave and Remain campaigns are "neck and neck" and the scale of victory in the referendum is likely to be "quite small" whatever the outcome, the Labour peer admitted.
Austerity sparked by Britain severing its links with Brussels would make the financial crisis look "like a walk in the park" and the poorest "will be the first to be hit and hit most", he said.
London's future "hangs by a thread" and public services will be the first in line for cuts as finances are hammered, according to the former European trade commissioner.
None of the alternatives to the current trading relationship with the EU would "give us so much" and "cost us so little" and renegotiating it would take between two and seven years, the peer told a business lunch staged by London First, which represents leading employers in the capital.
As a string of polls showed Leave nudging into the lead following intense campaigning on immigration by Brexit-backers, Lord Mandelson warned: "We will be taking the message across the country to Labour voters."
"There is so much riding on this, there is so much at stake."
"Do not be conned into thinking you can solve your concerns about immigration by leaving the European Union because all that will do is wreck the economy and make everything you rely on weaker and everything else you want in life harder to get."
The Labour former Cabinet minister said the EU was a "democratic club" that had a record of "opening up markets".
Tax receipts will fall if Britain leaves which will hit investment in national infrastructure and major decisions, such as airport expansion in the south east, will be kicked into the long grass, he claimed.
"The future of this great global city, in my opinion, hangs by a thread. If Britain votes to leave Europe a lot of Europe will have to leave London."
Quitting will mean the UK faces "bargain basement" World Trade Organisation rules, he said.
"Unless and until we reach a fresh agreement with the EU, and I judge that will take between two and seven years, we will be reliant on standard WTO rules," he said.
He claimed leading Leave campaigner Boris Johnson wants to do away with all trade tariffs, which would allow countries to fill British markets with "cheap, low wage" goods.
China will think "Christmas has come early", while Brazil will be "thrilled" and the Americans would "have a riot", he said.
"Far from being able to transfer savings from leaving the EU to more spending, for example, on the NHS, or schools, or universities, or farmers, we will eventually be facing cumulative, year-on-year post-Brexit austerity that would make the last financial crisis look like a walk in the park."
Lord Mandelson said the public did not understand the full consequences of quitting. "Why should they?" he said.
"The devil is in the sort of detail that virtually no one apart from grizzled old ex-trade negotiators would ever, ever have reason to know."