Image copyright Creative Commons Image caption Interserve holds the contract for cleaning Network Rail stations

Shares in Interserve - one of the UK's largest providers of public services - have collapsed after it revealed it is seeking a rescue deal.

The firm, which works in prisons, schools, hospitals and on the roads, has £500m of debts and says its rescue plan will involve issuing new shares.

Its shares initially fell as low as 6.5p, down 70% from Friday, but closed at 11.5p, representing a 53% fall.

A year ago, Interserve's shares had been worth 100p each.

On Sunday, the firm said it was "making good progress" on a recovery plan.

However, it added that current investors could see the value of their stake in the company slashed, in what it called a "material dilution for current Interserve shareholders".

Details of the plan, which it said it expected to announce early next year, are yet to be finalised.

The government has said it supports the company's long-term recovery plan.

On Monday, Interserve was awarded a new £25m Welsh public sector contract to help in the redevelopment of Prince Charles Hospital in Merthyr. Work is set to start this month.

Workers at the Foreign Office and the NHS are among Interserve's tens of thousands of UK employees.

Lenders resigned to write-offs

Simon Jack, business editor

Sources close to Interserve's creditors have told the BBC they accept they may have to write off some of their loans to ensure the company's survival.

Lenders described talks around the company's future as "extremely fluid".

They recognised inevitable comparisons to the situation that engulfed Carillion, but insist the company's management still has lenders' support.

The BBC also understands Interserve is expected to announce imminently it has secured new public service contracts.

The Labour party leadership - ideologically opposed to the role of private companies in the provision of public services - has insisted that no new government contracts should be awarded to the company while it is in a parlous financial position.

That position is widely considered within industry to be detrimental to the future of a company that employs 75,000 worldwide, 45.000 in the UK.

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Interserve said it was in talks about a "deleveraging plan", which it said would deliver "a strong balance sheet".

"Although the form of the deleveraging plan remains to be finalised, it is likely to involve the conversion of a substantial proportion of the group's external borrowings into new equity, an element of which may be sold to existing shareholders and potentially other investors," the firm said.

It would be second rescue deal for Interserve, with the company refinancing its debt in March.

Its troubles have been blamed on cancellations and delays in its construction contracts as well as struggling waste-to-energy projects in Derby and Glasgow.

Interserve claims its prospects are improving, and says it will increase profits this year.

What does Interserve do?

From its origins in dredging and construction, the company has diversified into wide range of services, such as health care and catering, for clients in government and industry.

At King George Hospital in east London, for instance, Interserve has a £35m contract for cleaning, security, meals, waste management and maintenance.

Its infrastructure projects include improving the M5 Junction 6 near Worcester, refurbishing the Rotherham Interchange bus station in Yorkshire, and upgrading sewers and water pipes for Northumbrian Water.

But Interserve is also the largest provider of probation services in England and Wales, supervising about 40,000 "medium-low risk offenders" for the Ministry of Justice.

In a statement, Interserve said: "The fundamentals of the business are strong and the board is focused on ensuring Interserve has the right financial structure to support its future success."

The company said its options included bringing "new capital into the business and progressing the disposal of non-core businesses ".

Interserve's difficulties follow the collapse of Carillion in January 2018, which put thousands of jobs at risk and cost taxpayers £148m.

Following that, the government launched a pilot of "living wills" for contractors, so that critical services can be taken over in the event of a crisis. Interserve is one of five suppliers taking part.

A Cabinet Office spokesperson said: "We monitor the financial health of all of our strategic suppliers, including Interserve, and have regular discussions with the company's management. The company successfully raised new debt facilities earlier this year, and we fully support them in their long term recovery plan."