LONDON, Dec 14 (Reuters) - Britain's BT on Wednesday unveiled a new wholesale fibre offer designed to encourage broadband providers like Sky, TalkTalk and Vodafone to move customers from slow copper connections to ultrafast end-to-end fibre.

BT, which is building Britain's biggest fibre-to-the-premises network, offered internet service providers (ISPs) discounts last year in return for connecting a majority of new lines to its fibre.

The scheme, called Equinox, was challenged by CityFibre, an alternative fibre provider (altnets), but regulator Ofcom decided not to intervene.

Equinox 2 broadens the scope of Equinox, which BT said had been taken up by about 49 ISPs.

Mark Shurmer, head of regulatory affairs at BT's network arm Openreach, said the new long-term pricing included speed tiers, with incentives to encourage the take-up of faster services.

It also had a "failsafe mechanism", which removed the theoretical possibility that Equinox 2 would stop an ISP choosing to use an altnet in a specific area, he said.

Chief commercial officer Katie Milligan said Openreach was keen to get more homes and businesses using its 15 billion pound fibre network as soon as it was built.

"To that end, we’ve responded to our customers' desire for lower prices and long-term certainty," she said.

"These offers don't commit them to Openreach exclusively but, alongside our new, faster speed tiers, we're confident they'll help them continue to support and delight their own customers in a highly competitive market."

Rival Virgin Media O2 said BT was facing the biggest competitive challenge in its history, with billions of pounds of fibre investment pouring into the UK, creating the prospect of genuine broadband wholesale competition at scale.

"To avoid putting planned and future investment at risk, and to safeguard fair competition, it's vital that these wholesale pricing proposals are thoroughly scrutinised to ensure Openreach is not using its market power and dominance to lock in providers and deter them from switching to other networks," said Virgin Media O2's CEO Lutz Schüler.

Ofcom said it would consider whether Equinox 2 raised competition concerns requiring intervention, adding it would expected to publish a consultation on its provisional view by early February.

Shares in BT rose 3% in early deals, topping the FTSE 100 index, supported by Goldman Sachs reiterating its 'Buy' rating, saying it "benefits from rising demand and fading altnet risks". (Reporting by Paul Sandle; Editing by Kate Holton and Mark Potter)