Baidu-backed ride-hailing service Yidao has lashed out at Tencent’s messaging service WeChat for blocking the ride service on their platform.

Founder of Yidao, Hang Zhou, penned an open letter to Tencent CEO Pony Ma on his public Weibo account this week accusing the social media platform of periodically blocking users from accessing the Yidao site through the social platform for competitive reasons.

Yidao competes directly with Tencent-backed Didi Chuxing, the country’s most popular ride-hailing service.

Users were unable to access the Yidao app from WeChat from July 13th. Following Mr. Zhou’s open letter, the ban was briefly lifted before being reinforced on July 14th. The ban appears to have been lifted again at the time of writing.

“I just do not understand why WeChat blocked the application,” he said, “Moreover, Uber and Shenzhou [UCAR] type apps have also been blocked, Didi is the only exception.”

Yidao recently introduced a feature that allows users to compare the cost of an Yidao ride with other rides.

Tencent released a public statement within hours of Mr. Zhou’s open letter saying that Yidao had been blocked by the site for asking users to share promotional material in return for cash rewards.

The scuffle highlights the fierce competition between China’s current top ride-hailing apps, which have been fighting a two-year long war of attrition fueled by subsidies and aggressive marketing campaigns.

In December WeChat blocked Uber on the platform, citing ‘malicious’ marketing tactics. The social platform has a range of rules that apply to businesses who wish to use the platform to market brands. Companies must have over 100,000 followers before they are able to advertise, and must also submit a relevant license.

WeChat claims Uber failed to submit the license, Uber fired back saying that they had the appropriate regulatory approvals but had never been asked to submit them. Baidu is a prominent investor in both Uber and Yidao.

Being blocked on WeChat is a serious blow for any company in China. The app, which boasts over 750 million total users with over 90% coverage in tier-one cities, has become a major marketing and communication tool for companies in China. The app not only supports public accounts, but a highly popular payment service, WeChat Pay.