The UK has run into trouble over its logo for the EU presidency after the Eurosceptic Bruges Group complained the swan emblem was similar to its own.
The UK's design team wanted to "avoid cliches"
The logo shows blue swans flying in a V-formation, but the Bruges Group also has swans flying in a V-formation.
The group, co-chaired by Lord Lamont, is threatening legal action to stop the logo being used unless the Foreign Office can explain the similarity.
The Foreign Office said any suggestion of plagiarism was "fanciful".
The Foreign Office says any similarity is "coincidental"
A spokesman insisted the design was original and had been registered as a trademark.
"Any suggestion that there was any plagiarism involved in the development of this design is completely fanciful," he said.
"Any apparent similarity between the UK Presidency logo and any other design is coincidental."
Former Chancellor of the Exchequer Lord Lamont has written to Foreign Secretary Jack Straw demanding an explanation.
The Bruges Group, which has Baroness Thatcher among its members, says he must adequately explain the similarity by noon on Friday.
Otherwise, the group, which has had its logo since the early 1990s, says it will consider applying for an injunction to stop the presidency logo being used.
Lord Lamont said: "Imitation is the sincerest form of flattery. Obviously, the government has realised that the Bruges Group has been right all along about the federal ambitions of the EU."
The government paid £30,000 to design agency Johnson Banks Design to come up with the logo, and said it represented "leadership, teamwork and efficiency".
Michael Johnson, of the design firm, said his team had been determined to avoid the "usual clichés", in particular stars.
Mr Johnson said that his design had passed "all trademark checks" before being adopted as the logo for the UK presidency.
"I'm afraid that Mr Lamont's (logo) seems to be too old and too unmemorable for anyone to know of it," he said.
"I would never profess to be the first person to have claimed a flock of swans as my own - that credit would surely go to mother nature."
The UK logo for its 1998 presidency was an arrangement of children's drawings of stars, unkindly compared by some to squashed pizzas.
The swans logo, unveiled on Thursday, was picked by a panel from the Design Council, Foreign Office, Cabinet Office and Department of Culture.
Bruges Group Director Robert Oulds said he much preferred his own body's logo.
"What the logo symbolised for us was the winds of change, changing how Europe worked from a bureaucratic monolith to a Europe of independent nation states."