Prosecutors are expected to announce on Wednesday whether Conservative politicians or officials will face charges for breaches of expenses rules.
The CPS is considering files sent by police following investigations in up to 27 constituencies.
It relates to claims some campaigning costs in the 2015 general election were wrongly recorded.
The Conservative Party has insisted administrative errors were to blame rather than any intention to deceive.
And it has said some of the spending was correctly declared.
It is alleged the Conservatives spent tens of thousands of pounds on local campaigns – including on “battle bus” visits by activists – which were either not declared or were wrongly registered as national spending.