Speaking in Parliament on Wednesday, UK Socialist MEP Richard Corbett said the institution had been a "gift" to these and other groups that he said sought to undermine the EU.
The veteran deputy, a former aide to ex-European Council President Herman Van Rompuy, has launched a book on the workings of the European Parliament.
He said, "The Parliament has come a long way since the first direct elections in 1979 and MEPs have had to fight hard for the powers it now enjoys.
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"But these powers and procedures have been used by others with a very different agenda."
He singled out Ukip and Marine Le Pen's Front National for particular criticism.
"We have to remember that these groups won seats in this parliament before any gains they made in national parliaments. But I am afraid that the European Parliament has been a gift for them.
“They have used, or should I more accurately say misused, this parliament and its procedures and resources.
"It is a platform for them to purely undermine the EU to their domestic audiences back home.
"They try to use Parliament to destroy the EU and we need to stand up to this and make a stronger case for the EU," said Corbett, a constitutional expert who regained his UK seat in the last European elections.
Corbett added, "Strangely, we see very little of many of them in Parliament, with Farage, for example, attending very few parliamentary committee meetings."
Corbett is a co-author of the ninth edition of book, 'The European Parliament', which he launched in Parliament's library.
He said, "Parliament has come a long way from being a consultative organisation, a 'talking shop' in effect, to what it is now: a co-legislator and the most powerful international parliament in the world."
Corbett said he first visited the Parliament as a trainee in 1976 - three years before the first direct elections.
He was the assembly's representative on the Lisbon treaty but will step down when the UK leaves the EU.
The book and Corbett's contribution to the Parliament were praised by Martin Schulz, who signalled out his efforts to keep the UK in the EU.
Schulz, speaking at the launch, said, "He is one of the fighters and, right now, we need people like him. Unfortunately, the EU referendum result means his time as an MEP will end relatively soon. His departure will be a big loss to this Parliament."
A spokesperson for the EFDD group, to which Ukip is affiliated, angrily hit back at Corbett's criticism of both Ukip and its leader Nigel Farage, telling this website, "Richard Corbett is an embittered loser and we don't care what he says about anything."
And a Ukip spokesperson said, "From May 2019, British MEPs will never attend the European Parliament again. This will be a source of great joy to Ulip MEPs. Richard Corbett is not doubt saddened by his future loss of income."