Grant, who supports tactical voting to prevent a Conservative majority, was photographed campaigning for Luciana Berger, the former Labour MP who is standing for the Lib Dems in Finchley & Golders Green, over the weekend.
In an attempt to capitalise on the endorsement, the Lib Dems tweeted it was “great to see Hugh Grant supporting @lucianaberger” and added: “Only the Lib Dems can take seats off the Tories - and together we can stop Boris and #StopBrexit.”
However, the British actor appeared to take issue with the claim that “only the Lib Dems” could defeat Conservative candidates.
“Your second para is not true,” he said in a retweet of the post.
Grant has backed Labour and Liberal Democrat candidates as part of his efforts to encourage tactical voting to defeat Boris Johnson.
The actor is not officially affiliated with any party and in 2015 said he was “not a Labour person, or a Tory, or anything else”.
Minutes after The Independent called the Liberal Democrats’ press office to ask about Grant’s tweet, the original post was deleted.
About half an hour later, Grant’s tweet was also deleted.
When asked about the post, a Liberal Democrat spokesperson said: "The Liberal Democrats are the only party that is campaigning to Stop Brexit.
"We are delighted that Hugh Grant joined Luciana and Chuka on the doorstep to take the fight to the Conservatives."
The embarrassment comes after a number of rows involving the party over accusations of “fake news” and misleading campaign material.
Last week, the party was criticised by the Society of Editors for producing political adverts that mimicked independent local papers.
“It is ironic how it is often politicians who complain about fake news but then set out to at least blur the lines for readers – and in this case voters – by packaging their partial messages to ape independent newspapers,” Ian Murray, the organisation’s executive director, said in his criticism.
Jo Swinson, the Lib Dem leader, has also been forced to defend her party’s use of bar charts in election leaflets after critics claimed they were misleading because they presented a close race between the Lib Dems and the Conservatives in some seats.
Layla Moran, a Lib Dem candidate and former maths teacher, admitted in November that “people do need to be careful about the bar charts”.
In some cases, campaigners have used the 2019 European parliament election results, in which the party came second behind the Brexit Party, as evidence that the Lib Dems have the best chance of defeating the Conservatives.
However, European parliament results are often unrepresentative of the results in general elections and 2019’s results go against recent national polling, which shows the Lib Dems in third place.
A polling average by Britain Elects on 2 December showed Ms Swinson’s party on 13.6 per cent, compared to 42.4 per cent for the Conservatives and 31.8 per cent for Labour – although the Lib Dems do perform better in certain constituencies.
After tweeting his criticism of the party’s post, Grant also retweeted a post from anti-Brexit campaigner Femi Oluwole that criticised both Labour and the Lib Dems for using the phrase “only our party” when talking about stopping Brexit or defeating the Conservatives.
“The only way to defeat Boris Johnson is via tactical voting,” Mr Oluwole said.
“Both Labour and the Lib Dems need to stop using that phrase because it undermines the basic premise of tactical voting.”