The parliamentary standards watchdog is investigating claims that the Conservatives have misused House of Commons facilities.
Two Labour MPs complained that several Tories, including leader David Cameron and shadow chancellor George Osborne, were using House of Commons dining rooms to raise party funds.
Under parliamentary rules, the private dining rooms cannot be used for "direct financial or material gain" or "as an inducement to recruit new members of outside organisations or non-parliamentary associations".
But Kevan Jones and John Mann have claimed that Tory patrons clubs are advertising dinners in these rooms as part of their fundraising campaigns, according to reports in The Guardian. Speakers at these dinners include prominent Tory MPs.
The office of Philip Mawer, the parliamentary standards commissioner, has confirmed that it has launched a preliminary investigation into the complaints.
Conservative chairman Francis Maude said he believed the party had acted within parliamentary rules, but welcomed Sir Philip's investigation as a way of clearing up a "grey area".
"If it were not possible for there to be any political dinners in the House of Commons then obviously that would be slightly absurd and the dining rooms would be empty half the time," he told BBC Radio Four's World At One.
"There is some ambiguity in the rules and Sir Philip Mawer will clear it up for us and we will follow whatever he says."
Speaking on Sky News earlier today, Mr Mann, MP for Bassetlaw, said he had documented 12 cases where Conservative associations of sitting MPs had been using the dining rooms of the Houses of Commons and Lords to fundraise for the party.
He warned: "Behind this are hundreds and hundreds more incidents of this happening. It's only the stupid few who've put it on their website that we've been able to document."
But a Tory party spokesman said: "The rules state that rooms at the House of Commons can be used for party political purposes and MPs may hire out the rooms for a charge.
"This appears to be an issue which affects all the political parties. We have asked Sir Philip to clarify the rules. If he has received an official complaint from Labour MPs then it is his job to investigate properly.
"We believe we've broken no rules and look forward to his conclusions."