Claims the Conservatives used a call centre in Neath to canvass voters during the general election campaign are being investigated by police.
The party previously denied it had broken electoral law by using the Blue Telecoms call centre.
South Wales Police said the probe was of “scale and significance” in a letter to Labour MP Wayne David.
The party said it was unable to comment on an ongoing investigation.
The Information Commissioner’s Office (ICO) also confirmed it was “currently investigating the Conservative Party in relation to a possible breach of Regulation 21 of the Privacy and Electronic Communications Regulations 2003 (PECR)”.
A Channel 4 News report earlier this year claimed the UK Conservatives contracted Blue Telecoms to conduct marketing campaigns ahead of the vote on 8 June.
The undercover investigation claimed the workers may have been carrying out paid canvassing, banned under electoral law, as they promoted key Conservative messages to undecided voters in the weeks before the election.
The report claimed calls were made to voters in key marginal seats, including Bridgend, Gower, Clwyd South and Wrexham.
The Conservative Party had said it did not break the law by using the company, which it said was hired to carry out legal market research and direct marketing.
In a letter to Mr David, South Wales Police said there was no timescale for the investigation because it is of “sufficient scale and significance that South Wales Police are unable to offer any”.
Mr David, Labour MP for Caerphilly, said: “I am pleased that both the police and the Information Commissioner’s Office are conducting detailed investigations.
“The allegations that the Conservative Party and Blue Telecoms broke electoral law during a general election campaign are extremely serious and the public need to have confidence in our electoral process. That is fundamental to our democracy.”
A spokeswoman for South Wales Police said: “South Wales Police can confirm it is investigating allegations relating to the Representation of the People Act 1983.
“As an investigation is underway, it would be inappropriate to comment any further at this time.”