The EU wants to force the UK to take a share of refugees.
Unlike most other EU member states there would be no legal requirement for the UK to take a "quota" of refugees if such a scheme were passed.
"Brussels, Germany and France want to force all EU nations—including Britain—to take their share of up to 160,000 refugees."
Daily Express, 4 September 2015
European leaders can't "force" the UK to take a quota of refugees from elsewhere in the EU, or anywhere else for that matter.
The reported EU proposal to relocate 120,000 asylum seekers from countries like Hungary, Greece and Italy appears to be on top of a plan first floated in May this year, which sought to distribute 40,000 people among other member states. This explains the reference to 160,000 by the Express.
Such a scheme might compel most member states to accept more asylum seekers, but the UK is an exception. We have what's often referred to as an opt-out, although strictly speaking what happens is that EU laws on the likes of asylum and border control don't apply to the UK unless we choose to opt in.
It left the UK out of its calculations for what it considered a fair scheme for relocating asylum seekers within the EU, and similarly seemed to accept that Britain couldn't be forced to resettle people from outside the EU either.
Relocation versus resettlement
The various proposals advanced recently appear to be for internal relocation from EU countries most affected by recent migration, rather than resettlement from countries around Syria.
The President of the European Council, Donald Tusk, seems to be describing internal relocation in referring to the need for 100,000 refugee places. So does the UN High Commissioner for Refugees, António Guterres, who wants 200,000 places.
This would be a temporary change from the current system of new arrivals being concentrated in the country they first arrive in, as the rules usually demand. The French and German leaders appear supportive of this concept, as the Express reported.
These relocation schemes for existing asylum seekers are different to proactively bringing in more refugees from countries near Syria, as the Prime Minister is considering. Mr Cameron's plans are a voluntary effort on the part of the UK.
Update 10 September 2015
We added more details about the European Commission's September proposals on relocation of asylum seekers.