The Electoral Commission has taken legal action to force the Conservative Party to disclose information related to its investigation into claims that it breached election spending rules.
The watchdog said it had applied to the High Court for a "document and information disclosure order" after party chiefs failed to provide requested details.
Conservatives are accused of recording the costs of activists bussed into marginal seats under individual candidates' limits, rather than as part of the national campaign.
Nine police forces have also launched fraud investigations.
The Commission said the party failed properly to act on two statutory notices requiring it to produce information - providing "limited disclosure of material in response to the first notice (issued on 18 February 2016) and no material in response to the second notice (issued on 23 March 2016)" - despite being granted extensions to the original deadlines to comply.
Bob Posner, director of party and election finance and legal counsel, said: "If parties under investigation do not comply with our requirements for the disclosure of relevant material in reasonable time and after sufficient opportunity to do so, the Commission can seek recourse through the courts.
"We are today asking the court to require the party to fully disclose the documents and information we regard as necessary to effectively progress our investigation into the party's campaign spending returns."
The party acknowledges that some accommodation for the activists was not properly registered, but insists that the bus tour was part of the national campaign organised by Conservative campaign headquarters (CCHQ) and as such did not fall to be recorded in individual constituency spending limits.
The case has led to calls for the new Police and Crime Commissioner (PCC) for Devon and Cornwall to stand aside while she is investigated over her part in the campaign.
Alison Hernandez, who was elected PCC last week, was the agent for Conservative candidate Kevin Foster in Torbay, one of the constituencies involved.
A Conservative Party spokeswoman said: "We advised the Electoral Commission on 29th April that we would comply with their notices by 1pm today – and we will do so.
"There was no need for them to make this application to the High Court."
The Commission said the extended deadline for both orders - each of which seeks separate information - expired on Monday.
A spokeswoman said it would examine any details provided later to check whether they satisfied all the demands.