The adjective intact, meaning (1) remaining sound, entire, or uninjured or (2) having all physical parts, is one word. It has always been one word, all the way back to its roots in the Latin intactus (which translates literally to untouched). The two-word phrase in tact is nonsensical, as the noun tact has nothing to do with being whole or uninjured, and its origins are different from intact‘s.
The hyphenated in-tact is also incorrect.
Examples
Mistaken use of in tact in place of intact is rampant, even in edited publications—for example:
While the interior is in tact, the menu is updated and there’s now WiFi, too. [Wall Street Journal]
Core buildings (which house the reactors) have to remain in tact for at least 40 years because dismantling them would release more radioactive material. [BBC News]
He said his players could hit the ice baths with their reputations not only in tact but in many case, enhanced. [Sydney Morning Herald]
The good news is that the correct intact is more common than in tact. These writers spell the adjective correctly:
There are few scenarios where I see the structure of our political parties remaining intact after the vote in May. [Globe and Mail]
The 5-4 decision left intact an Arizona tax subsidy that was enacted because the state constitution forbids direct aid to religious schools. [NPR]
Part of my daily route to work took me along a Roman pavement, its original stones intact. [Telegraph]
