A Conservative MP who tweeted about the products of a company she received a £10,000 donation from "needs to be investigated", a former MP has said.
Fay Jones registered the donation from the Radnor Hills water company at the start of January.
Elfyn Llwyd, a former Commons standards committee member, said her social media posts about the firm come near to a breach of the rules.
Ms Jones denied there was anything "untoward" in the posts.
Meanwhile the owner of the company said he had not asked her to promote the products and said the MP should be applauded for supporting local firms.
On 11 February, on Facebook and Twitter, the newly elected MP for Brecon and Radnorshire referred to the company's products being on sale in the UK Parliament.
She posted that she was "delighted to see Parliament stocking Radnor Hills water".
The MP registered a donation of £10,000 from the water and soft drinks bottling company Radnor Hills, which is based in her constituency in Knighton, Powys, on 8 January.
The law states that MPs are allowed to receive donations from "permissible donors", including companies registered in the UK.
Delighted to see Parliament stocking @Radnorhills water! A brilliant firm in my constituency - employing over 200 people and putting sustainability right at the heart of their business. #RadnorHills pic.twitter.com/K7rOTn8pKq— Fay Jones MP (@JonesyFay) February 11, 2020
Her Facebook post said: "Radnor Hills is a fantastic firm based in Knighton which manufactures a range of drinks.
"It employs over 200 people and puts environmental sustainability right at the heart of its business plans.
"I was delighted to go and meet the owner William Watkins in November.
"His is a model to follow and I am determined to support businesses which give so much back to our local communities. Well done Radnor Hills!"
The code of conduct for MPs states "no member shall act as a paid advocate in any proceeding of the House".
Paragraph 11 of the code states: "The rules on lobbying are intended to avoid the perception that outside individuals or organisations may reward members, through payment or in other ways, in the expectation that their actions in the House will benefit that outside individual or organisation, even if they do not fall within the strict definition of paid advocacy."
Mr Llwyd, who was a member of the House of Commons' committee on standards and privileges, said: "I think it comes very near to breaching the rule against paid advocacy.
"Looking in particular at the Facebook entry that she had, it's clear there that that Facebook photograph was taken on the House of Commons premises. I think it should be investigated.
"She is advocating for a company in her constituency - nothing wrong with that, everybody does that.
"But having received a £10,000 donation from that company beforehand, to my way of thinking there is something that needs to be investigated because that can't be right."
The ex-Plaid Cymru MP added: "Left unchecked, I think it would be a big mistake because this could be opening up the floodgates."
Thrilled to see that Parliament is stocking Radnor Hills water - direct from Radnorshire! Radnor Hills is a fantastic...Posted by Fay Jones - Member of Parliament for Brecon and Radnorshire on Tuesday, February 11, 2020
Ms Jones told BBC Wales: "There is nothing untoward here. I was just pleased to see a product from a business in my constituency being sold in Parliament and I wanted to offer my support."
In a statement, William Watkins, founder of Radnor Hills, said the donation was made to Ms Jones' local party association rather than her directly, before she was elected.
Mr Watkins added: "I did not, and have not since, asked her to promote or advertise our products.
"Her tweets therefore were in no way something we had suggested to her, or expected from her, and to suggest anything other is slanderous.
"In fact she should be applauded for recognising and supporting companies in her area, whoever they are," he added.