Oftel said it made the order to "encourage greater competition and lower prices in the provision of high speed, 'always-on' internet lines to business customers".
BT must make a number of improvements to its wholesale leased line products to promote greater take-up by other operators, said Oftel.
Britain, ranked number 22 in a recent 30-country survey by the Organisation for Economic Co-operation and Development (OECD), has fallen behind in the race to popularise high-speed internet.
Boosting broadband
However, Oftel earlier this week said Britain was beginning to move up the broadband league, helped by price cuts to consumers since April.
Oftel said it was ordering BT to make improvements after completing the first phase of an investigation into allegations of barriers to competition.
The order covers leased lines, which provide an 'always-on' service between, for instance, two buildings belonging to one firm, as well as links known as partial private circuits (PPCs).
PPCs are the key wholesale components of leased lines, which enable other telecoms firms to use BT's network to deliver services to their own customers.
Greater competition
BT opened up the cheaper, wholesale version of leased lines to rival telecoms providers in August last year, after a ruling from Oftel.
The move has boosted competition in the leased lines market and allowed other firms to cut costs "significantly", the watchdog said.
But the switch-over to the cheaper version has been slowed by concerns about the terms on which BT has offered the wholesale product, according to Oftel.
"At the moment many operators' circuits have not been transferred to the wholesale products.
"I want to see a far greater take-up to ensure that consumers see the full benefits of competition in the leased lines market," said Oftel director general David Edmonds.
He said the decision was an important part of Oftel's work to boost the development of broadband in the business market.