Shipping firms won a five-year extension to an exemption from some EU competition rules but European Commission chief José Manuel Barroso denied it was a favour to a friend on whose luxury yacht he holidayed.

Mr Barroso and his family spent six days on the yacht of Greek shipping, oil trading and banking tycoon Spiro Latsis last August, after he had been confirmed as the next head of the EU executive Commission but before he took office.

EU lawmakers have criticised Mr Barroso for making the trip, saying he could have been lobbied by the Greek tycoon.

"The fact that a commissioner's friends and acquaintances may be affected by Commission policy does not in itself represent a conflict of interest," Mr Barroso wrote in a letter to the European Parliament president, obtained by Reuters.

The former Portuguese Prime Minister oversaw the decision to give shipping firms another five-year exemption from anti-cartel rules, enabling them to work together in cargo transport.

EU Competition Commissioner Neelie Kroes withdrew from the case because she used to sit on the board of P&O Nedlloyd, a shipping company.

"I do not even know whether the interests of Dr Latsis or his family are directly affected by these regulations, which as you know, do not concern individual undertakings but the sector as a whole," Mr Barroso said in the letter.

The heads of political groups in the EU assembly are studying the letter and will decide at their weekly meeting tomorrow whether to take any further action, a parliament spokesman said.

Socialist floor leader Martin Schulz called on Mr Barroso, who led a centre-right party in Portugal, to explain himself to party leaders when the affair became public last week but no other group has followed him in demanding a public hearing.