The decision follows a campaign, which ran this summer, warning customers that if they left British Gas, they would no longer have access to its engineers.
The ASA ruling will criticise British Gas' use of the advertising for its "undue appeal to fear". It is also expected to forbid the company from using it again, although the campaign has run its course.
The activity, which used the line "Walk away from British Gas and wave goodbye to our engineers", led to several of its competitors, including Npower and Powergen, uniting to condemn the campaign as "scaremongering".
In a statement, British Gas defended its advertising claims, and said it referred to its On-call Assistance service, available to any British Gas customer. It accused its rivals of "nit-picking".
British Gas has lost more than 500,000 customers this year, having imposed three price increases as a result of rising wholesale energy prices.
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