From the OED:
The preposition onto written as one word (instead of on to) is widely used, but is still not wholly accepted as part of standard British English. It is also important to note the distinction between the preposition onto or on to and the use of the adverb on followed by the preposition to: she climbed on to (or onto) the roof but letís go on to (not onto) the next point.
INTO ē preposition 1 expressing motion or direction to a point on or within somewhere > The train went into the station.
IN TO [adverb + preposition] : In order to collect her work, she came in to the office.
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