Agencies creating, selling and hosting Scotland's increasingly notorious tax haven firms are turning over at least £25m a year, The Herald has calculated.

There are now dozens of businesses in the UK and overseas profiting from a new industry providing "brass plate" shell companies called Scottish limited partnerships or SLPs.

Some such firms have been implicated in multi-million-pound tax evasion and corruption cases, including the alleged laundering of $1bn from Moldovan banks last year.

Read more: Revealed - the secret bases of Scotland 's 25,000 hidden tax haven firms

Oxfam this week launched a campaign to reform SLPs after The Herald revealed they were being openly advertised as "zero-tax Scottish offshore companies" throughout the former Soviet Union.

The union which represents tax inspectors, the Public and Commercial Services Union, yesterday through its weight behind the international aid organisation. Lynn Henderson, its national officer, said: "We welcome the launch of Oxfam’s campaign and urge all MSPs to sign up.

"We are committed to a fair and progressive tax system, to which tax havens are a threat."

SNP, Labour, Scottish Greens and Liberal Democrat politicians have all already called for Westminster action on SLPs amid concerns that they are tarnishing Scotland's international reputation.

Nobody knows exactly how much dirty money has gone through the accounts of SLPs. However, it is possible to estimate how much they cost to run.

Read more: Revealed - the secret bases of Scotland 's 25,000 hidden tax haven firms

A Herald analysis of all the roughly 25,000 SLPs currently registered in Scotland suggests the scale of the mushrooming but murky industry which creates and maintains such shell firms.

Some 17,000 SLPs currently have obviously opaque ownership, usually though another layer of shell companies registered in traditional tax havens in the Caribbean.

Adverts in the former USSR for off-the-peg companies show that a typical off-the-peg SLP costs around £1500, often including a certificate of good standing or other documents to enable their purchaser to open a bank account in the European Union, typically in Latvia.

The start-up or maintenance fees of the roughly 17,000 most obviously opaque SLPs would amount to some £25m a year. This figure does not include SLPs registered by legitimate Scottish law firms providing tax-efficient vehicles for international investors.

Read more: Revealed - the secret bases of Scotland 's 25,000 hidden tax haven firms

There were just under 6000 SLPs registered in the financial year to April 2016. Of these more than 5500 were from a score of company creation agencies churning out multiple SLPs with secret owners. Registrations of SLPs continue to boom, hitting a record 498 a month in the last financial year.

Some 14,000 SLPs - as The Sunday Herald revealed last month - are registered at just 15 addresses. It can cost as little as £23 a month to buy an address for an SLP at one of Scotland's virtual office complexes, where companies are registered but have no physical presence. Executives at such PO Box providing companies insist they have no way of knowing if a company registered at their address is breaking the law.

There is no suggestion that firms which create, market or host SLPs are aware of any criminal activity carried out by SLPs.

However, The Herald has now named numerous SLPs involved in alleged internet scams or high-profile corruption cases in the former Soviet Union, including one against Ukraine's alleged "arms Mafia" and, yesterday, one linked with an allegedly corrupt MP, also from Ukraine.