UKIP leader Nigel Farage has been accused by Labour of "anti-Royalism" for refusing to take part in a standing ovation for Prince Charles.
Prince Charles addressed an audience of MEPs in Brussels
Mr Farage said his sit-down protest was due to the Prince of Wales advocating an increase in EU powers in a speech to MEPs on climate change.
The Prince's advisers were "naive" to allow him to make such a speech during the row about Europe's future, he said.
But Labour MEP Gary Titley said he was "disgusted" by Mr Farage's behaviour.
In his Brussels speech, Prince Charles described the fight against global climate change as a war, with the Doomsday Clock "ticking ever closer to midnight".
He said it needed the biggest public private partnership in history to solve the crisis - along with greater leadership from Europe.
"Determined and principled leadership has never been more needed. Surely this is just the moment in history for which the European Union was created," he told MEPs.
'Huge political row'
The Prince received a standing ovation from about 150 British MEPs and representatives from the European Parliament's climate change and environment committees.
But Mr Farage - who joined the standing welcome for the Prince but made a point of remaining seated after his speech - said the Prince's advisers had been "naive and foolish at best" to allow him to make a political speech at such a delicate time.
"We are in the middle of a huge political row over Britain's future in Europe and into Brussels waltzes Prince Charles to say 'well done everybody but you can do better'.
"It reminded me of young Mr Grace in that old TV series 'Are You Being Served'.
"How can somebody like Prince Charles be allowed to come to the European Parliament at this time to announce he thinks it should have more powers?
"It would have been better for the country he wants to rule one day if he had stayed home and tried to persuade Gordon Brown to give the people the promised referendum."
UKIP, the Conservatives, some Labour and Lib Dem MPs and MEPs, want a referendum on the EU treaty, saying it is the same as the EU constitution on which there had been a referendum promised.
The government says there is no need for a referendum because the treaty does not have the same constitutional implications as the EU constitution.
Labour's Gary Titley said he had been "embarrassed" by Mr Farage's behaviour.
"As a British citizen I was deeply honoured to be present when the Prince of Wales made such an important speech to members of the European Parliament.
"I was embarrassed and disgusted when the Leader of the UK Independence Party, Nigel Farage remained firmly seated during the lengthy standing ovation Prince Charles received."
He added: "I had not realised Mr Farage's blind adherence to right wing politics involved disloyalty and discourtesy to the Royal Family.
"He should be thoroughly ashamed of himself and should apologise to the British people he represents."