The Cambridge-based company was set up by a consortium of licensed taxi and private hire firms to create an app allowing customers to compare fares and pickup times and then book their selected cab.
But the service was promoted by sending unlawful direct marketing texts inviting customers from some of the cab firms which had invested in the new venture to download the app.
Between May 27 and 5 June 5 2016, 165 complaints were made to the spam text reporting service run by mobile phone industry body the GSMA concerning unsolicited marketing texts sent by Cab Guru. A further complaint was made direct to the ICO using the Online Reporting Tool.
Cab Guru said it had carried out a one-day text marketing campaign using numbers provided by five of its shareholder firms. A total of 706,650 messages were sent, of which 360,373 were received.
However, the company could not provide any evidence that the customers had consented to being contacted by Cab Guru in this way.
ICO enforcement group manager Andy Curry said: "Just because an organisation might have a person's mobile phone number in its records, that doesn't mean it can call or send them marketing messages without their consent. This also applies to any associated companies.
"It also doesn't matter whether you do this for a single day or every day for a year – most mobile users hate receiving unsolicited spam texts and the ICO will continue to take action against firms which send them."