The Government has been accused of failing to prioritise the danger posed to national security and businesses from losing data sharing rights with the EU after Brexit.
Data experts say Britain will have to wait up to three years to be granted an "adequacy decision" from Brussels, threatening to stop the flow of data immediately unless a temporary deal can be struck.
At stake is access to information sharing through the Europol law enforcement agency and to the Schengen information system, which holds an 8,000-name watchlist of potential terror suspects.
In the absence of an over-arching deal, separate agreements would have to be struck with individual police forces and intelligence services - with the danger that vital information will “fall between the cracks”, one expert told The Independent.
“Those organisations would need to be persuaded that Britain can be trusted with data. At the very least, it's likely to be disruptive and uncertain,” said Ruth Boardman, a partner at the international law firm Bird & Bird.
“The worry is that government simply doesn't have this issue on its radar as a high enough priority - for both the economy and law enforcement.”
The warning was echoed by Antony Walker, the deputy chief executive of the trade association techUK, who said: “The danger of an ad-hoc fix is that things fall between the cracks and, for security issues, that would be a significant risk.
“Ministers say this is on their list of ten priority issues, but I'm not sure they understand the full significance of the threat.”
Without a temporary agreement, companies will have to move part of their operations to the EU or risk losing business to rival firms on the continent, they said.
Committee in dark
A Home Office minister, Baroness Williams of Trafford, left a House of Lords committee in the dark when she gave evidence on the controversy in April.
She was unable to say what “the transition arrangements might look like”, telling peers: “I am not being unhelpful. It is just that I cannot.”
Asked whether Britain was willing to sign up to amendments to data transfer rules after Brexit, to ensure continued compliance, Baroness Williams replied: “I literally do not know. That is to be determined.”
The threat arises because, after Brexit, the UK will be treated as a "third country" - requiring the European Commission to be satisfied it will protect data to the same degree as EU members.
Both Boardman and Walker warned that securing the "adequacy decision" would take at least two years, from March 2019, on the basis of such negotiations in the past - making a temporary deal the only option.
However, the European Court of Justice (ECJ) has ruled that mass data retention of the kind allowed in the UK under the Investigatory Powers Act, or “snoopers' charter”, is unlawful, throwing up a further hurdle.
A Conservative spokeswoman told The Independent: “We have been very clear that as part of the Brexit negotiations, we want to agree a deep and special partnership between the UK and the EU to ensure continued cooperation on security. Theresa May is the best person to secure that arrangement.”