Letters are to be sent to more than 90 hoteliers as part of a campaign against city drug dealers.
Police in Portsmouth, who believe visiting drug dealers often stay in local hotels and bed and breakfasts, want hoteliers to be more vigilant.
Establishments in the city have been told to look out for a number of suspicious signs including payment by cash and last minute bookings.
Hoteliers have also been asked to be aware of London home addresses.