LONDON — A former investment banker and a businessman were convicted of insider trading on Monday in what the British authorities have described as the largest crackdown on improper trading in Britain.

After a three-month trial in Southwark Crown Court in London, Martyn Dodgson, a former banker at Morgan Stanley, Lehman Brothers and Deutsche Bank, and Andrew Hind, a businessman and trained accountant, were convicted of conspiracy to “insider deal,” as insider trading is described in Britain.

The conspiracy began in November 2006 and ended in March 2010, prosecutors said.

The case sprung out of an investigation called Operation Tabernula. The investigation began with a series of raids by the British authorities in March 2010

Three other men who also were on trial were acquitted of criminal charges on Monday.

“This was an extraordinary and complex case of a type not prosecuted in this country before,” Mark Steward, the Financial Conduct Authority’s director of enforcement and market oversight, said in a news release on Monday.