You'll no longer be able to use your companion certificate for Delta One

Delta has long offered one of the most rewarding credit card perks for customers with one of the airline's Amex cards: an annual companion certificate, letting you bring a fellow traveler along on a trip for a significantly reduced fee. Unfortunately, Delta recently began enforcing a long-standing restriction, limiting access to certain lie-flat seats.

As outlined by the carrier's terms and conditions, customers with the Delta SkyMiles® Reserve American Express Card and Delta SkyMiles® Reserve Business American Express Card get a companion ticket that can be used for most cabins, including first class.

Customers with the Delta SkyMiles® Platinum American Express Card and Delta SkyMiles® Platinum Business American Express Card get a certificate that can be used to cover coach travel.

All companion tickets can only be used on round-trip tickets within the contiguous United States. Residents of Alaska, Hawaii, Puerto Rico or the U.S. Virgin Islands can originate from those destinations to travel to the 48 contiguous states, based on the address attached to your SkyMiles account.

Officially, the Reserve certificate has always been valid for economy, Comfort+ and first-class travel, but travelers have also been able to book companions onto select Delta One flights, scoring a lie-flat seat within the U.S.

Over the past few days, we've heard from a number of flyers who weren't seeing the flights they were expecting pop up when completing a search with their certificates attached — Delta One has stopped appearing on any route that offers the carrier's top product.

We reached out to Delta for clarification — a spokesperson confirmed that Delta One had previously been available, and noted that tickets will be honored as booked.

"With the Delta SkyMiles® Reserve American Express Card, Card Members receive a Companion Certificate each year upon Card renewal. With this Certificate, they can invite someone special to fly with them on an eligible round-trip domestic flight in First Class, Delta Comfort+ or Main Cabin. We are aware of certain routes that may have been able to book in Delta One and we will honor those tickets."

Unfortunately, following Delta's confirmation, it seems that the airline's going to be sticking to the terms and conditions a bit more closely going forward — certificate holders may have been able to book Delta One in the past, but I wouldn't expect that to be a possibility anymore.

There are a number of other restrictions as well — specifically, lower-level fare classes will need to be available, as outlined by Delta below:

"Tickets are only available in I and Z classes of service for First Class travel, and only available in L, U, T, X, and V classes of service for Main Cabin travel. For Delta Comfort+ travel, tickets are available in W and S classes of service, but only when L, U, T, X, or V classes of service are available in the Main Cabin."

We've also had trouble finding eligible first-class flights on many routes — if you come across a flight that meets the fare class requirements, you may want to call Delta's reservations line at (800) 221-1212 to see if an agent can book your companion trip. Meanwhile, economy and Comfort+ should be a bit easier to come by:

It's worth noting that cardholders and their companion are eligible for upgrades on these tickets, though, including to Comfort+, first class and Delta One. Also, if you manage to book an economy ticket, you could always confirm a Delta One upgrade using a Global or Regional Upgrade Certificate, if it's available.