Asked when the Tories would respond, the prime minister’s spokesperson said there are “no plans” currently in place to launch the party’s own campaign.
“I’m sure if we have one we’ll let you know about it,” he told journalists, prompting laughter.
The spokesperson also declined to say for certain that a manifesto would be published, but implied one was likely to be, saying: “Those kinds of things will be, I’m sure, set out in the usual and familiar way.”
The confusion comes as Ms May continues to insist the European elections can be avoided, although experts told The Independent it was already too late to ratify any deal in time.
Mr Farage only launched his Brexit Party two weeks ago, but one poll put its support at 27 per cent – well ahead of both Labour (22 per cent) and the Tories (15 per cent).
A staggering 40 per cent of Conservative councillors said they would back the former Ukip leader’s party, in a growing grassroots revolt against the prime minister.
One local Conservative group has even taken the extraordinary step of refusing to help with campaigning, insisting the elections “should not be happening”.
A crushing defeat in the European elections will trigger renewed pressure for Ms May to quit in order for a summer leadership election to take place.
But, asked about preparations for a Conservative campaign, her spokesperson said: “There are no plans that I should be outlining now.”
He also denied the compromise talks with Labour were set to fail, insisting: “There is energy and urgency in delivering the Brexit that people voted for.”
Standing in for Ms May at prime minister’s questions, David Lidington, her de-facto deputy, warned movement was needed from both sides if the cross-party talks were to succeed.
“The substance and the tone of those conversations between the government team and the opposition team have been constructive,” Mr Lidington told MPs.
“I think there is a genuine attempt to try to find a way through but I’m not going to hide the fact that this is very difficult, because – if it’s going to work – it’ll mean both parties needing to make compromises and us ending up with a solution that, unlike any other so far proposed, will get a majority in the house.”
On Tuesday, Ms May accused Labour of dragging its feet in the talks – but Jeremy Corbyn has protested that the prime minister is refusing to shift on her red lines.
The two sides seem unlikely to even discuss the key controversy of a customs union before next week, underlining the stalemate.