Image copyright SPL Image caption Doctors are unsure how many patients have been killed by carbapenemase-producing enterobacteriaceae.

Hospitals in England are not required to officially report infections of a "superbug" capable of resisting our most powerful antibiotics, a BBC investigation has found.

Cases of "carbapenemase-producing enterobacteriaceae" (CPE) have shown a sharp rise.

Public Health England said a lack of mandatory reporting made assessing the true extent of the problem difficult.

But Chief Medical Officer Sally Davies said laboratories did share data.

CPEs are bacteria that have the ability to make an enzyme which can fight off carbapenem antibiotics - drugs usually prescribed by physicians when patients have serious infections unresponsive to other antimicrobial medicines.

Until 2007, monitoring authorities were seeing only three to five samples per year from referrals by hospital labs across the UK.

This is a severe risk to humanity that will build if we do not take action Dame Sally Davies, Chief medical officer, England

But numbers have risen sharply since then.

Eight hundred samples of CPE were sent to Public Health England (PHE) laboratories by hospitals for analysis in 2012.

'Mandated referral'

Thousands of patients elsewhere in the world, including in countries with advanced health care systems like America, have died from untreatable infections.

In England, hospital trusts have a legal duty to report cases of MRSA and C. difficile, but these have seen a significant decline following measures to control them, enforced by the government.

No similar obligation exists for CPEs, despite concern these organisms present a greater threat.

The head of PHE's antibiotic resistance laboratory, Prof Neil Woodford, said an "accurate handle on the extent of the problem" was needed.

He said: "I think we do need to move to a system for mandated referral and for active surveillance of these highly resistant organisms.

"If the carbapenems fall, there are very few antibiotics left in reserve. We need to take steps now to ensure that we have carbapenems that remain active for future generations."

Central Manchester University Hospitals NHS Foundation Trust revealed it was seeing about 10 cases a week of people who are carriers of CPEs.

The Trust has an active monitoring programme for CPEs, having become, in 2009, the first in England to have an issue with these "superbugs".

It has stepped up infection prevention and control measures.

Since then other hospitals across the UK have had similar cases.

Antibiotic apocalypse Image copyright SPL Image caption Some bacteria are becoming resistant to our best drugs This might read like the plot of a science fiction novel - but there is genuine fear that the world is heading into a post-antibiotic era James Gallagher, Health and science reporter, BBC News Read more from James

Not everyone gets ill because the drug-resistant bacteria can be present harmlessly in our gut.

But carriers can infect others if the microbes get into a wound.

'Severe risk to humanity'

A File on 4 investigation also raised questions about whether the authorities could put a figure on the numbers of patients who have died following CPE infections.

The programme was told the matter is complex because if people who are very poorly with other conditions also get an infection, it is sometimes not easy to tell which has been responsible for their death.

Public Health England said it did not record mortality figures.

These latest revelations come just a few weeks after Prof Dame Sally Davies, England's chief medical officer, published dire warnings in her annual report.

She told File on 4 that antibiotic-resistant infections presented a "severe risk to humanity".

She said: "This is a ticking time-bomb and it is terribly important that we take action now."

But she added that compulsory surveillance of CPEs has not so far been necessary.

She said: "I believe that we have the information we need."

The government is shortly expected to announce a five-year plan which will include improvements to the way the NHS conducts superbug surveillance, and better gathering and analysis of data.

Listen to the full report on File on 4 on BBC Radio 4 on Tuesday, 21 May at 20:00 BST and Sunday, 26 May at 17:00 BST. Listen again via the Radio 4 website or the File on 4 download.