ITV News has seen a document that was sent to more than 9,000 customs officials late on Friday night.
It states that officers “should encourage completion” of the public health passenger locator forms but should “not enforce” it.
The guidance makes it clear that not everyone passing through customs will have their forms inspected.
At the border, officers should “spot check a certain percentage of arriving passengers which will be agreed on a regional or port basis with National Operations Headquarters”.
There are 42 categories of people who are exempt from the obligation to self-isolate for 14 days on arrival in the UK, but only diplomats are exempt from completing an online declaration form.
Border officers are advised to use their discretion when checking forms: “A fisherman will not have information such as a booking reference or a seat number; a lorry driver may sleep in their cab and thus not have an address in the UK.”
But the guidance admits that officers will not have access to the original electronic record that the passenger filled in and “are therefore not able to check that the record is genuine”.
According to the guidance, “errors may be corrected” and fines should only be imposed if a passenger provides “manifestly false information without a reasonable explanation, (for example stating that their name is Mickey Mouse or that their address is Buckingham Palace) and the passenger refuses to resubmit credible information".
How does the new system work?
At the weekend, the CEO of Ryanair, , told the Mail on Sunday that the quarantine rules are “useless” and joked that “most holidaymakers will get away with writing things like "Mickey Mouse, 1 Walt Disney Street” on their arrival forms.The guidance suggests that anyone who does this risks being fined £100.
Last week, Lucy Moreton, from the Immigration Services Union (ISU), said the system the government has introduced is “farcical” and that passengers could fill-in “complete rubbish”.