A weed I presume, never seen this before

I've been mowing my lawn for 30 years, more or less controlling dandelions and clover, but this is a new one one me this spring. A few weeks ago, the entire lawn and borders started sprouting these plants, I've probably been the victim of a airborne seed attack, there are far too many to treat individually. They sprout with two opposite plain leaves, getting to about 50mm high. Then the next two leaves to appear are a completely different shape. I've tried searching for various ways of saying 'different leaves', to no avail. 'Lawn weeds' as a search is also next to useless, it doesn't seem to be visible in the 'top 16'.

In this specimen, the first two plain leaves have already suffered a bit of pest attack.

Any ideas? Is it relatively benign, or a triffid?

 

It looks like a Sycamore seedling, just been pulling some out of my garden.

 

It certainly is, maybe a neighbouring tree has just reached maturity, or perhaps an already mature one in a nearby garden managed to spread its seeds your way in and unseasonable gale?

 

Im currently battling my way through these its driving me insane. I can clear an area of them and then two days later there is more

 

you can usually tell that it is a tree if it is very hard to pull the seedling out.

 

Thanks all. Yes, it does look like sycamore, doesn't it. I didn't think 'tree' as the stalk is not at all woody, at least at this height, not as woody as an ash seedling of this size.

I already do a (semi) regular ash patrol along my borders, so adding sycamore to the hit list is not going to tax me unduly. I'm pleased to read that mowers generally do for those in the lawn. So, panic over.