Business Secretary Vince Cable has referred the state-backed Royal Bank of Scotland (RBS) to the City regulator amid renewed allegations over its treatment of struggling business customers.

Sky News understands that Mr Cable has passed to the Financial Conduct Authority (FCA) a dossier of evidence compiled by Lawrence Tomlinson, a businessman who also advises the Department for Business, Innovation and Skills (BIS).

Among a string of claims made by Mr Tomlinson, according to people familiar with the dossier, is that RBS has engineered the transfer of a significant number of business customers into a specialist division in order to profit from the higher fees it can charge.

Run by veteran executive Derek Sach, the global restructuring group (GRG) is the division of RBS which manages the bank’s problem loans.

Since the financial crisis led to it being rescued by taxpayers in 2008, RBS has become one of the biggest property owners in Britain through West Register, another arm of the bank.

Mr Tomlinson, who works for the Government under the title entrepreneur-in-residence, is said to have uncovered evidence that the taxpayer-backed bank sought to exploit distressed small business customers by accelerating their move into the unit.

The more intensive supervision of companies when they enter the work-out groups of banks – when, for example, they are in danger of breaching borrowing agreements – results in them being charged higher fees.

Mr Tomlinson is also understood to repeat an earlier criticism that banks such as RBS frequently appoint favoured accountancy firms to oversee the work-out process, resulting in the outcome desired by the lender but which sometimes entails companies being placed in administration.

RBS has already said that it will investigate the activities of the GRG division in response to a highly critical report on its SME lending practices published earlier this month by Sir Andrew Large, the former deputy governor of the Bank of England.

It is unclear whether the alleged misbehaviour by RBS amounted to a formal breach of the City regulator’s rulebook.