Treasury sources confirmed that Illsley had been granted the ceremonial post of Crown Steward and Bailiff of the three Chiltern Hundreds of Stoke, Desborough and Burnham - the traditional way of resigning from Parliament.
Labour is planning to trigger a by-election in his Barnsley Central seat on March 3.
Illsley is due to be sentenced on Thursday after admitting dishonestly claiming £14,000 of expenses on January 11.
He could theoretically have stayed on as a MP with a jail term of less than 12 months.
However, following heavy pressure Illsley expressed “deep regret” over his actions and said he would quit before the court decided his fate.
He is believed to have been receiving his £65,000-a-year parliamentary salary over the past month - meaning he will have pocketed roughly £5,400 since being convicted.
Labour said it expected to move the by-election writ on Thursday, and the poll will be held on March 3.
Despite the questions over his expenses, Illsley was re-elected to the seat in May with a majority of more than 11,000.