He said: "We introduced these clients to Scottish Widows, and now it seems they are trying to take the client from us.
"It makes you question what else they might fail to honour or what they might sacrifice just for short-term gain?"
But a spokesman for Scottish Widows said the provider had always recommended that customers seek financial advice before making decisions on their finances.
The spokesman said: "[We] explain that customers who already have an IFA should contact him or her in the first instance, which has happened in this case as a result of the letter.
"Not all customers will have an IFA, so we share a number of ways they can find a suitably qualified one, including via Unbiased, to allow them to make an informed decision that best suits their needs."
Alan Chan, IFA and director of IFS Wealth & Pensions, said Scottish Widows should know which clients already have an adviser and which do not and send out appropriate letters to suit each customer.
Mr Chan said: "I've not seen such a letter or had any clients make me aware of this.
"If it was sent to advised clients too, I would be very unhappy with Scottish Widows.
"I can understand that it is a one-size-fits-all templated letter to all personal pension clients, advised or direct, but they should know which clients have IFAs and which do not.
"So if they are urging clients who already have an IFA to review their pensions, they should just encourage them to contact their adviser directly and have removed that paragraph about using Scottish Widow's own, restricted, advisers.
"Talk about biting the hand that feeds them."
Dippy Singh is a freelance journalist
