Three Plaid Cymru MPs have been found guilty of misusing a parliamentary allowance.
The parliamentary standards commissioner announced on Monday that Elfyn Llwyd, Adam Price and Hywel Williams used their communications allowance to pay for
double-page adverts in local newspapers in the week before the Welsh assembly elections in May.
Sir Philip Mawer recommended that the nationalist MPs repay the taxpayer around £5,000 each.
His findings were backed by the Commons standards and privileges committee which agreed that the adverts were "likely to be seen not as reports to constituents but as material promoting Plaid Cymru ahead of the election".
Communications allowances of up to £10,000 per year were introduced to allow MPs to update constituents on their parliamentary activities, but are not intended to be used for political ends.
The Electoral Commission had already ruled in October that the adverts should be declared by Plaid as Welsh assembly election expenses.
The MPs accepted the conclusions but insisted they had acted in "good faith", with the spending signed off by the Commons Department of Finance and Administration (DFA).
"We accept the ruling of the parliamentary commissioner and the standards committee," Plaid parliamentary leader Llwyd said. "My colleagues and myself will comply fully with it.
"It should be noted, however, that several matters complained of were dismissed by the commissioner.
"Having worked closely with the House of Commons Department of Finance and Administration during the production and commissioning of our parliamentary reports we are, however, disappointed with the outcome. We maintain that we have acted in good faith throughout, and fully in line with the advice that was offered to us by the DFA at the time of the publication of the reports.
"We note the commissioner's view... 'Whatever the committee's conclusion about whether or not to uphold the complaints, I believe that the experience of these cases points to the need for further consideration of the rules of the communications allowance.'"
Officials at the DFA told Sir Philip that they did not expect the text to be so widely circulated in the press.
And one of the three Welsh Labour MPs who complained about the adverts, Ian Lucas, said: "The communications allowance was brought in to enable better understanding of the work of parliamentarians and it undermines all MPs if it is misused.
"To use taxpayers' money for electoral campaigning is plain wrong.
"To vote against the allowance, then use it mere weeks later to fund co-ordinated advertisements, knowing they would run during a closely-fought assembly election campaign, brings the allowance into disrepute.
"The MPs involved have now been told by both the Electoral Commission and the Commons that they misused their allowances and I hope they will not only pay the money back but also do the honourable thing and apologise."
