Economic immigrants from recently acceded EU member states have been highly beneficial to Britain's economy, a House of Lords report says.
The upper house's EU committee says that the addition of a "high-skilled, low-cost" workforce from eastern Europe has allowed UK companies to compete with their Asian counterparts.
Today's report into proposed future enlargement of the EU adds that in countries where quotas were placed on immigrants, many workers were forced into the black market.
Home secretary John Reid has already indicated that Romanian and Bulgarian migrants wishing to work in Britain will be subject to quotas when the two states join the EU in 2007.
More than half a million workers have arrived from eastern Europe, the majority from Poland, since the 2004 round of EU enlargement.
And peers say that future expansion should be welcomed rather than feared by existing member states, as enlargement is an "integral part" of the EU's continued development, bringing "tangible benefits" to all parties.
Despite admitting that an enlarged EU will require a degree of institutional reorganisation, the committee says that membership could help "fragile and fractious" countries in the Balkans forever turn away from sectarian violence.
Croatia, Turkey and FYR Macedonia are all candidates to join the EU, with Albanian, Bosnia Herzegovina, Serbia and Montenegro all likely possibilities further into the future.
Lord Grenfell, the House of Lords EU committee chairman, today said: "The last round of enlargement brought countries from eastern Europe into the EU and all the evidence suggests their inclusion has had a beneficial effect on their own economies and those of the established EU members."
He went on to say: "The prospect of EU membership for the western Balkan states can act as a key olive branch to encourage its population and politicians to move away from corruption and sectarian violence and towards a future of integration with the rest of Europe. The risk of the Balkan region becoming destabilised again and the pressure this would place on the EU and western European countries should motivate current member states to keep the door open to the western Balkans."