LUXEMBOURG, Oct 26 (Reuter) - European Union fisheries ministers unanimously agreed measures on Thursday to control fishing in Western waters, despite a warning they were too weak and might lead to trouble.

The measures were the final step before Spain and Portugal controversially gain greater access to prized fishing grounds off the west coasts of Britain and Ireland next January, seven years earlier than agreed when they joined the EU in 1986.

Unanimous agreement was needed after member states rejected proposals by EU Fisheries Commissioner Emma Bonino, which they said were too bureaucratic and costly.

"The compromise was a balance between efficiency, simplicity and non-discrimination," Luis Atienza, Spanish Fisheries Minister, told a news conference after the meeting.

The compromise was based on a three-tiered Spanish EU presidency plan, backed by the main fishing countries -- Britain, Ireland, France, Portugal and Spain -- involving more flexible and less stringent fish reporting requirements.

Bonino said fishing controls were up to member states but warned that the weaker measures may lead to trouble next year.

British and Irish fishermen strongly resent the intrusion of Spanish vessels and there have been violent clashes in the past.

"It could in future lead to further conflict," Bonino told the news conference.

But she added that she was prepared to put the new system to the test. "It's in no one's interest to go into a vacuum in January 1996...I prefer to have something than nothing." The new measures were warmly welcomed by Britain.

"We have very successfully concluded arrangements for western waters," British Fisheries Minister Tony Baldry said.

In national waters, member states will fix their own measures and for trips of up to 72 hours, vessels over 15 metres will only have to report their fishing plans before leaving port. Vessel log books will record catches and movements.

Full reporting requirements -- on entry and exit from port and fishing zones -- will only be required for fishing trips of more than 72 hours into other member states' waters.

"The Commission's proposals were incredibly cumbersome," Baldry said, adding that skippers would have spent most of the time on the phone reporting their movements.

After a furious debate, fisheries ministers agreed last December to allow 40 Spanish vessels into most of the Irish box -- a protected fisheries zone around Ireland.

British and Irish fishermen complained that scarce fish stocks would be plundered while the Spanish said they were being discriminated against.