Failing to secure a Brexit deal with the European Union would put Britain at a "very big disadvantage", Hillary Clinton has claimed.
The former US presidential challenger said no deal would put pressure on businesses and the disruption could be "quite serious".
Britain must not pin its hopes on a US deal because Donald Trump "doesn't believe in trade" and was on the verge of pulling out of existing agreements with its biggest trading partners, she warned.
Mrs Clinton told BBC One's The Andrew Marr Show: "I think it would be a very big disadvantage to Britain.
"I mean, no deal meaning no preferential trade deals, which means products in Britain would not have the kind of easy access to the European market that you've had under EU membership.
"It could very well mean that there would be more pressure on businesses in Britain, if not to leave completely, at least also have sites and employment elsewhere in Europe.
"I think that the disruption for Britain could be, you know, quite serious."
President Trump has said it is "possible" the US will pull out of the North American Free Trade Agreement (NAFTA) struck more than two decades ago with Mexico and Canada.
Asked about the UK's trading future with America, Mrs Clinton said: "Well, yes, but you're making a trade deal with someone who says he doesn't believe in trade. So I'm not quite sure how that's going to play out over the next few years.
"He looks like he's on the verge of taking (us) out of NAFTA rather than reworking NAFTA. Our biggest trading partners in the world are Canada and Mexico. So these will have real world economic consequences."
The former US secretary of state accused Leave campaigners of giving "fabricated" information to voters during the referendum and raised the role of data mining and analysis company Cambridge Analytica.
An investigation was launched by the Information Commissioner's Office into the firm after complaints the Leave.EU campaign, backed by ex-Ukip leader Nigel Farage, had not declared its role in its campaign.
Mrs Clinton said: "Looking at the Brexit vote now, it was a precursor to some extent of what happened to us in the United States. Whatever the role Cambridge Analytica played for example. But the amount of fabricated, false information that your voters were given by the Leave campaign."
She added: "You had, you know, Mr Farage campaigning for Trump and the like. You know, the big lie is a very potent tool, and we've somewhat kept it at bay in western democracies, partly because of the freedom of the press.
"But we've now developed advocacy press. You know, obviously there have always been newspapers who leaned right or leaned left and they kind of counterbalanced each other.
"But given the absolutely explosive spread of online news and sites that have sprung up that are very effective at propagating false stories, we've got some thinking to do ... but there has to be some basic level of fact and evidence in our politics. Well, frankly, in all parts of our society."
The prospect of no deal has become increasingly in the spotlight after talks in Brussels continued to be deadlocked.
European Commission president Jean-Claude Juncker has said the Brexit process will take "longer than we initially thought", blaming delays on Britain's failure to settle its financial obligations.
The EU's chief Brexit negotiator, Michel Barnier, said talks on issues including the "divorce bill" had not made sufficient progress for him to be able to recommend moving on to the second phase of negotiations, covering trade.
Chancellor Philip Hammond branded the EU the "enemy" but rowed back on the comments within minutes.
Sir Martin Donnelly, who quit his role as permanent secretary for the Department for International Trade (DIT), earlier this year, warned severing ties with the European Court of Justice would have a "major and lasting negative effect" on jobs and investment .
He suggested MPs could vote to keep the UK in the single market after Britain leaves the EU.
Sir Martin told BBC Radio 4's Today programme: "That is a choice for Parliament to make. Ultimately the decision about our future legal framework with Europe will have to be made by Parliament."
No deal could create a "huge amount of legal uncertainty" for businesses, he warned.
"If people aren't sure what the regime is going to be with Europe then not only will it have a chilling effect on investment here it also makes it more difficult to do good trade deals around the world," he said.
"Am I worried about our future choices? Yes I am because if we chose to leave the single market access we have got today it will be very difficult and very slow to negotiate something even not nearly as good in the years ahead."