Passengers arriving at Britain's busiest airport are being let off over customs rules on duty free allowances for cigarettes and alcohol, a watchdog report indicates.
Officers who discover people arriving from outside the European Union with undeclared goods in excess of limits should seize the items.
But Border Force staff at Heathrow felt a "zero tolerance" approach could "alienate members of the travelling public, leading to more confrontations", according to a report from the Independent Chief Inspector of Borders and Immigration.
An inspection team observing the green "nothing to declare" channel for those arriving from outside the EU witnessed "discretion" being exercised in two cases.
In one the passenger was marginally over the alcohol limit and was allowed to proceed.
In the other instance two women were found to be carrying excess cigarettes and tobacco. "One of them was elderly, and they were allowed to move into the red channel and pay the duty on the excess goods," the report said.
People arriving from outside the EU are allowed a certain amount of goods without paying duty or tax. Allowances include 16 litres of beer, 4 litres of wine and 200 cigarettes.
Instructions to staff state that when a passenger is intercepted arriving from a non-EU country and is found to be carrying undeclared alcohol or tobacco goods in excess of their duty free allowance, they must "seize all of the goods of the same type as the excess irrespective of volume or quantity".
Inspectors found that all of the Border Force managers and staff interviewed were aware that it was unlawful to exercise discretion in the area.
However, some felt there were circumstances in which it would be appropriate to do so - such as where a passenger held amounts "marginally" over the allowance or where elderly or vulnerable travellers were encountered with goods in excess of allowances.
Chief Inspector David Bolt's report said: "Officers stated that they were told by managers to target large seizures, and were afraid of becoming tied up dealing with low-level seizures and missing high-risk passengers or flights.
"They said that the paperwork involved in a seizure of 200 cigarettes was the same as for a seizure of 200,000 cigarettes. Some managers and staff questioned the cost effectiveness of making smaller seizures."
In group interviews most managers and staff said they should be allowed to exercise discretion when discharging their powers. Comparisons were made with police who do not serve speeding notices for driving at 31mph in a 30mph zone, the report said.
It concluded: "Border Force senior management had moved to ensure that officers in the customs channel complied with the law.
"However, the 'zero tolerance' approach was not supported in principle by some of those working in the channels, and not applied in practice in all cases."
Overall, the inspection found that the Home Office had acted upon recommendations made in a previous report on Heathrow.
Mr Bolt said: "I am pleased that standards in Border Force's operations were raised as a result of the recommendations this inspectorate made in 2015. But some areas require further attention and senior managers will need to do more to achieve the necessary improvements. "