Police checkpoints will be set up on the border between Gloucestershire and Wales as the country starts a two-week lockdown to stem coronavirus rates.
Gloucestershire Constabulary said it will stop people travelling into Wales and encourage them to turn around if they "are not satisfied with their explanation".
If they carry on regardless, they will inform Welsh police, who can fine them.
The force said the vast majority of people were abiding by current rules.
Rates are relatively low in Gloucestershire, with 62.3 per 100,000 people currently infected with coronavirus.
They are higher in Monmouthshire, where 130.6 per 100,000 are infected. In Cardiff, that rate increases to 329.8 per 100,000.
The restrictions are set to be introduced in Wales from 18:00 BST until 9 November.
Pubs, bars and restaurants, gyms and all non-essential shops will be closed and supermarkets have been told they can sell only essential items.
The nationwide "firebreak", which comes after local lockdowns in communities, aims to slow the rise in Covid cases and hospital admissions.
A spokesman for Gloucestershire Constabulary told the Local Democracy Reporting Service (LDRS): "While we cannot issue fines to those travelling from Wales into the county we can inform the host force of those we stop about what has happened so they can take action.
"Officers will be running an operation that will cover routes from Wales into the Forest of Dean and if we stop someone travelling from Wales we will be engaging with them to find out why, explaining the legislation and encouraging them to turn around if we are not satisfied with their explanation."