Andrea Leadsom's Conservative leadership campaign has rejected claims she retweeted a remark that Britain was "overrun by foreigners".
The energy minister came under fire from supporters of fellow contender Stephen Crabb and others over the message.
Leadsom formally launched her Tory leadership bid this morning and is hoping to be the pro-Brexit alternative to Theresa May.
The tweet, apparently sent on June 20, was first highlighted by Guardian reporter Claire Phipps.
But a spokesman for Leadsom told The Huffington Post UK: "We have conducted a detailed investigation into the circumstances of this tweet. Andrea Leadsom did not retweet the message from @QueenBritain beginning 'I really doubt'.
"It appears that the message in question never existed, but was fabricated in some way. We are continuing to investigate how the impression was created that Andrea retweeted this message. We will not speculate about those circumstances."
On Tuesday, the spokesman added the campaign was not implicating Phipps in the anyway. "At no point did we think let alone suggest that Claire Phipps had any part in the fabrication of the message. We were not asked about her role. Had we been. We would have made it clear at that point," the spokesman said.
The retweet was criticised by the @CrabbForBritain Twitter account which backs work and pensions secretary Stephen Crabb's Tory leadership bid. The Crabb campaign has said the account is not official.
A spokeswoman for the Crabb campaign told The Huffington Post UK of the @CrabbForBritain account: "This Twitter account has absolutely nothing to do with us. It is not verified and we have had no contact with them and they have had no contact with us".
And the Crabb supporting Twitter account told HuffPost UK: "We are not official we are a grassroots organisation supporting Stephen, kinda like a British SuperPAC (alas without the money)."
The @CrabbForBritain account has also criticised May in recent days.
Labour MP Jonathan Reynolds said "a lot of irresponsible rhetoric was used in the referendum campaign, and we have seen the consequences of that".
"If Andrea is reconsidering things that she said or did, she should say so frankly and help bring the country back together - not try and distance herself from it quietly hoping people don't notice," he told HuffPost UK.
During her speech today, Leadsom said EU nationals who currently live in the UK must not be "bargaining chips" in Brexit negotiations.
This morning, foreign secretary Philip Hammond said it would be "absurd" to guarantee them the right to remain in Britain for as long as they liked if the same right was not given to UK nationals living in the EU.
Hammond's comments matched those of May - who he is backing to succeed David Cameron as leader.
Leadsom said: "I commit today to guaranteeing the rights of our EU friends who have come here to live and work."