Texas billionaire R Allen Stanford is being investigated by US authorities over the activities of his Houston-based Stanford Financial Group and his bank in Antigua, connected to allegations about payments on certain financial instruments, according to media reports.

Several federal agencies, including the Securities and Exchange Commission, the FBI and the Internal Revenue Service, have spent “many months” examining Mr Stanford’s affairs. His bank issues high-yielding certificates of deposit.

The focus of the investigations is on how the bank could issue CD’s that pay interest rates that are more than twice the national average.

A spokesman for Stanford Financial said the company had been told by the SEC and by the Financial Industry Regulatory Authority, a securities industry oversight group, that “their visits to our offices were part of a routine examination.”

Following the alleged $50 billion alleged Ponzi scheme that US money manager Bernard Madoff is suspected of orchestrating, regulators are turning up the heat on money-management firms that appear to be performing significantly better than their peers.

This is not the first time Stanford’s business operations have raised eyebrows.

Stanford, a diversified financial firm that offers a broad array of services, including investment banking and research, holds about $8 billion in deposits at its bank and has about $50 billion in assets in its wealth management affiliate, according to its spokesman.

However, a wrongful-termination suit filed in a state court in Texas last summer alleges the asset sizes may have been inflated. The two former Stanford brokers who filed the suit said they had left the firm amid fears they could be implicated in the various “unethical and illegal business practices” they claim to have witnessed.

In their suit, they claim Stanford overstated the asset value of individuals in order to mislead potential investors, failed to file mandatory forms disclosing its clients’ offshore accounts, and purged electronic data from its computers in response to an SEC investigation.

Stanford, which filed a countersuit against the two men seeking repayment of certain loans, denied the men’s accusations.