Comcast Corp. said that starting this week, it is doubling the speeds of two of its existing Internet speed tiers in certain markets, including Greater Boston, at no additional cost to home subscribers.

Xfinity Blast customers will now get download speeds of up to 50 million bits per second; previously, they could get 25 million bits per second, Comcast said. Extreme 50 customers will now receive speeds of up to 105 million bits per second. The previous high was 50 million bits per second.

According to the company, this is the seventh time since 2002 that it has increased speeds for customers.

Comcast also disclosed plans to launch Xfinity Platinum Internet, a new ultrafast speed tier, throughout markets in its Northeast division. Xfinity Platinum Internet offers download speeds of up to 305 million bits per second.

Comcast and its rivals frequently boost the broadband speeds they offer in their pursuit of subscribers. But while the extra speed can pay off for households with multiple users, it can be overkill for some consumers.

Verizon Communications Inc. last month launched FiOS Quantum, the ultraswift broadband service of up to 300 million bits per second.