A return by BT to the mobile consumer market may well have to wait until the first quarter of the UK incumbent’s next financial year (beginning 1 April) as it continues to wrestle with technology designed to seamlessly hand voice calls and data traffic from 4G to Wi-Fi once customers are in range.

The Daily Telegraph reported BT’s difficulties, citing an unnamed source. “The handover is absolutely essential for BT to do what it wants to do with mobile. It’s possible the launch will slip into the second quarter of next year now.”

Using Wi-Fi is an important way for BT to keep its costs down. Under an MVNO deal with EE, BT leases 4G spectrum from the UK’s largest mobile operator to fill coverage gaps between its some 5.4 million Wi-Fi hotspots. The less traffic running over the leased spectrum, reports the Daily Telegraph, the less BT has to pay EE.

BT had wanted to launch a mobile consumer service in its current financial year, and a BT spokesman told the Daily Telegraph that plans were still on track.

Niall Dunne, BT Group’s chief sustainability officer, said in April that the UK fixed-line incumbent was looking toward the “back of the year” for a consumer 4G proposition.

There’s plenty of speculation that BT is keen to develop a quad-play strategy, bundling mobile with broadband, phone services and BT Vision. (BT has spent lavishly on TV rights for English Premier League football and other sporting events in an attempt to lure BT Vision subscribers).

BT already offers 4G for business customers. BT Business unveiled its first 4G mobile plans for UK SMEs in August, which don’t carry a premium over 3G data allowances.