Celebrations took place in new member states

A report published by Trade and Industry Secretary Patricia Hewitt said the 10 new states would bring investment and export opportunities.

Street parties and fireworks in the newcomer states marked the moment the EU expanded to 25 members at midnight.

It makes the EU the biggest trading bloc with a population of 455 million.

The nations joining the EU are: Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia.

Enlargement offers great opportunities for UK companies Patricia Hewitt Trade and Industry Secretary Irish step up security for EU visit Benefits row over EU entry

Prime Minister Tony Blair will join a ceremonial summit of all 25 EU leaders on Saturday in Dublin, chosen as the venue because Ireland holds the EU presidency.

Security was expected to be tight with Irish media reporting that 4,000 officers would be patrolling the streets with some 2,500 soldiers also said to be on stand-by.

Reports that a "hardcore" group of up to 300 international troublemakers was planning to target Dublin were dismissed by protesters as scaremongering, and an attempt to deter peaceful anti-globalist, anti-capitalist protesters.

Ms Hewitt's report says trade with the 10 expansion countries is growing faster than the UK's trade with the world as a whole.

And enlargement means more opportunities for UK firms to bid for infrastructure projects in those countries, it claims.

Mr Hewitt said: "From today, the EU becomes the largest single market in the world.

'Unfounded fears'

"Enlargement offers great opportunities for UK companies.

"When the EU last expanded there were fears about jobs and the economy for existing members which never materialised.

"I believe similar concerns this time will prove to be equally unfounded."

Parties were thrown at midnight

People from the eight Eastern Europe accession states who start work in the UK will have to register with the Home Office.

With each registration certificate, a TUC advice leaflet will be sent out offering guidance on their employee rights and to combat exploitation.

TUC General Secretary Brendan Barber said: "Workers from Poland, Estonia, Latvia and the other accession states need to be as well-informed as they can be on work issues to lessen the chance that they will be shoddily treated by unscrupulous employers."

Mr Blair used an article in the Times newspaper on the eve of enlargement to welcome the new members saying the expansion was "good for Europe and good for Britain."

He reiterated his warning that "no one will be able to come from the new accession countries to Britain simply to claim benefits" or be eligible for housing "unless they are economically active".

But he stressed: "I am equally determined that we continue to welcome those whose skills, talents and energy we need".

Conservative shadow foreign secretary Michael Ancram said: "As the European Union grows, its diversity of cultures, histories and traditions will grow. Diversity is a strength we should cherish."