The health secretary has approved plans to downgrade the first hospital using failure regime legislation. However, the downgrade is subject to conditions, the details of which remain unclear.

Jeremy Hunt backed the recommendations of the special administrator appointed to South London Healthcare Trust, the most contentious being that the emergency and maternity services at neighbouring Lewisham Healthcare Trust be downgraded.

He also ordered the £447m-turnover South London Healthcare be dissolved, split into three, have its historic debt paid off and receive government support for its private finance initiative payments.

Mr Hunt told the House of Commons he had accepted the proposals on the condition that Lewisham would still have an accident and emergency with “24/7 senior medical cover”, rather than the urgent care centre proposed by special administrator Matthew Kershaw.

The size of the unit, the conditions it will treat, its cost and the effects of the downgrade on co-dependent services at Lewisham are not known.

The health secretary said he had made this condition after receiving advice from NHS medical director Sir Bruce Keogh. He said: “[Sir Bruce] believes that overall these proposals, as amended, could save up to 100 lives every year through higher clinical standards.”

But this figure does not appear in the five-page letter Sir Bruce sent to Mr Hunt setting out his clinical advice on the reconfiguration.

The “100 lives” figure does appear in Mr Kershaw’s final report, but it is not clear how Mr Hunt came to attribute it to Sir Bruce in the Commons or whether Sir Bruce agrees with the figure. The Department of Health said Sir Bruce “did not undertake additional analysis” of the 100 lives figure but that his support for the principle of fewer units “implicitly” meant he endorsed it.

Senior medics at Lewisham Hospital have challenged the research on which the number is based.

The research, produced for NHS London in September 2011, compared mortality rates with a survey of what surgical facilities trusts had for admitted emergency patients.

However, the medics said this research had failed to take account of the fact that patients admitted at weekends tended to be more ill when it compared weekend with weekday mortality rates at London hospitals.

The Lewisham doctors pointed out in their letter that their trust performed well under the standard hospital mortality indicators.

Definitions of what constitutes an A&E department are not clear but the NHS Commissioning Board last month appointed Sir Bruce to take over a DH project on the specification and “nomenclature” of A&E units.

Mr Kershaw made his initial report in October 2012 after Andrew Lansley placed South London Healthcare Trust in the failure regime last July, the first time the powers in the 2006 National Health Service Act have been used.

The special administrator said the scope of his recommendations needed to be opened out wider than just South London Healthcare because the financial problems could not be solved within the organisation.

Mr Hunt’s announcement was welcomed by senior figures in the NHS, one of whom described it as “brave, considering we put him between a rock and hard place”.

Lewisham council said on Tuesday morning it was close to beginning legal action.