Councils are pouring millions of taxpayers' pounds into commercial property to make up for money lost by government cuts.
Norwich City Council is snapping up £40m-worth of shops and offices and spent £13.4m last year on buildings to rent to businesses last year.
It has not told the public what it has bought or for how much.
But research by this newspaper reveals the council bought a block of shops on London Street for £9.4m in April.
And late last year it splashed out £6.8m buying Lloyds Bank on Gentleman's Walk.
More big deals are in the pipeline. The council's cabinet is meeting on Wednesday to approve a large investment, but the details are not being made public.
Green Party councillor Denise Carlo said there should be a public debate about whether commercial investments were the best use of taxpayers' money.
'Councils are being starved of funds by the government and it is pushing them down these risky routes,' she said.
'There is so much uncertainty around, especially for retail with big names going to the wall.
'It has become a bit of a trend around the country but it really is an unknown risk.'
MPs on the Public Accounts Committee warned in 2016 of the risk to councils of putting huge amounts of cash into commercial property, particularly with many high streets struggling.
'If commercial decisions go wrong, council taxpayers will end up footing the bill and other services will be under threat,' they said.
But Norwich City Council said it had long invested in commercial property and other councils across the country were also doing it. Its purchases are reviewed by expert consultants.
The council also denied it was investing 'heavily' in commercial property - despite the £40m it is putting in representing around one-third of its annual income.
So far it has funded the purchases through its own money but will have to borrow in the future to fund other deals.
It snapped up 16 Gentleman's Walk, where Lloyds Bank is, late last year.
The council paid £500,000 over the guide price for it but it will get more than £400,000 a year in rent until 2026, giving it a return of 6pc a year.
It also bought the row of shops on London Street housing Laura Ashley and Mountain Warehouse, for £9.4m late last year.
Commercial confidentiality means the council keeps its purchases secret.
However, even after the sale is complete, it has not publicised any of the details.
Its public list of property it owns has not been updated for three years.
The council has now said it will be updated this autumn.
In November last year councillors were asked to approve an extra £15m of spending on commercial property as the current budget of £10m that year was not enough.
In a report justifying the spending, the council's chief finance officer Karen Watling said the rental income was of 'increasing importance' in helping the council pay for public services.
It is holding about £32m in cash reserves and the council said it was getting around 4pc a year from its recent purchases rather than 0.6pc it would get in a bank.
A spokesman said one reason the council was investing in commercial property was because of low interest rates.
'By using cash holdings like this we get a higher rate of return than we can achieve by depositing the cash in banks and building societies, plus we unlock rental income from the properties,' a spokesman said.
The council said it aimed to get a 2pc annual return on its property investments.
'The council has owned commercial property for decades so this is not anything new for us,' they added. 'As of March 2018 the council own 277 commercial properties. We already therefore have experience of buying, selling, maintaining and managing such properties.'
While Norwich buys and lets commercial buildings, South Norfolk is building the office space itself and then renting it.
It built an office block called Crafton House at Poringland for £1.88m in 2016.
It is now being rented out and is 80pc full, giving the council a return of just under 6pc a year.
Councillor Michael Edney, deputy leader South Norfolk Council, said: 'By investing directly the council has more control over the returns it makes on its investments, rather than relying solely on investing in third parties or putting cash in the bank.'
Breckland Council has also invested in commercial property - although much smaller sums than Norwich.
It bought land on Brocks Road, Swaffham, last year for £444,000 where it now rents industrial units.
Councillor Adrian Stasiak said: 'The money we raise through investment helps us to be financially self-sustaining, reducing our reliance on national funding and our vulnerability to future cuts.'
He said the rental income from its 200 properties - of around £2.5m a year - was more than it got through council tax.
Waveney Council, meanwhile, refused to say if it had bought any commercial property or if it had plans to buy any.
Land Registry records show it bought 18 properties and land parcels in 2017 and 2018 - more than any other council in the region.
•£14m for RAF Coltishall
Norfolk County Council said it had looked at but decided against buying commercial properties and renting them out.
Its major investment has been the old RAF Coltishall site which it has turned into Scottow Enterprise Park.
The council bought the site for £4m in 2013 and is putting in £10m to revamp it.
It effectively made a £274,000 profit for the council last year from renting space to businesses - excluding the £6m it poured in to refurbish it that year.
A council spokesman said: 'Scottow Enterprise Park does return an income for the Norfolk County Council but it also creates a number of jobs and business in this area.
'It contributes to the regeneration of the area and opened up employment opportunities when RAF Coltishall closed.
'This is an investment towards future growth and jobs in Norfolk which is vital for the county to continue to thrive.'
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