European police would have "free rein" to spy on Britons and demand their DNA under plans being considered by the Home Office this week, campaigners have said.
The power would allow prosecutors from any European Union country to place people under surveillance, bug telephone conversations, and monitor bank accounts.
It could also lead to disproportionate requests, with demands for the DNA of British tourists if there was a serious crime in a resort they had visited.
The UK must decide whether to opt in or out of the European Investigation Order (EIO) by July 28, the Home Office said.
Jago Russell, chief executive of campaign group Fair Trials International (FTI), said: "Police officers from Portugal to Poland would have free rein to demand recordings of our private conversations, copies of our bank statements and even our DNA.
"The UK Government has rightly said it will support new EU justice laws that make us safer and respect our civil liberties. The problem is, these proposals do neither."
An FTI report found the proposals would make it possible for Spanish police investigating a murder in a nightclub to demand the ID of every British citizen who flew to that country in the month the attack took place.
It could also demand that UK police search the DNA database to see if any of those identified are on it, and to provide the DNA records of any such person to the Spanish authorities.
Even if the UK police wanted to refuse the request on the grounds that it would impose "a huge cost and resource burden and it risks sending Spain the DNA profiles of people never charged with an offence", they would not be allowed to under the EIO proposal, the report said.
A Home Office spokesman said: "The Government is currently considering whether or not we should opt in to the European Investigation Order."
