Mr Boswell was criticised after he took an £18,000 interest-free loan which he received as a result of his job as a contract manager.
There is no suggestion Mr Boswell has done anything illegal but he has admitted the arrangement and said it was “common practice” in his previous industry.
However, Mr Boswell has now been reported to the commissioner, Kathyrn Hudson, over claims he failed to declare his roles as a company director – a potentially serious breach of House of Commons regulations.
The House of Commons register of interests exists to allow members to disclose financial interests outside their parliamentary work
Rules state that MPs must disclose any companies in which they have shareholdings above 15 per cent. Official records from Companies House show that Mr Boswell is the sole director or Johnston Ltd. His register of interests entry does not mention the company. The complaint was sent to the commissioner by Paul McGarry, a Liberal Democrat candidate for Central Scotland in next year’s Holyrood elections.
Ms Hudson will now decide whether to launch an inquiry into the complaint.
If an inquiry is launched and finds that there has been a breach of the rules, the MP could face sanctions ranging from having to apologise to the House, to being suspended from the Commons or even expelled.
First Minister Nicola Sturgeon has come under growing pressure to condemn Mr Boswell after she said tax avoidance was “awful” and “despicable” earlier this year.
Scottish Labour MSP James Kelly called on the First Minister to publicly criticise Mr Boswell, SNP MP for Coatbridge, Chryston and Bellshill.
Mr Kelly said: “These are serious allegations and the SNP must look into this as a matter of urgency. If politicians are saying one thing in public you would expect them to be living up to it in private. Nicola Sturgeon branded tax avoidance obscene, immoral and despicable. She called for a zero tolerance approach in Scotland.”
Scottish Liberal Democrat peer Jeremy Purvis called for the parliamentary commissioner to investigate what he said “appears to be a clear breach of the code of conduct” by Mr Boswell.
He said: “The First Minister has previously said that we must be “unequivocal about how obscene and immoral and downright wrong” it is to avoid tax. The First Minister cannot remain silent over these allegations.
“People will think that her words on tax avoidance were utterly meaningless unless she acts.
“It also seems that Mr Boswell has failed to register his business interest with the House of Commons, and failed to declare that financial interest in debates.”
An SNP spokesman said: “Phil Boswell has declared all interests in line with the rules, including all relevant income.”