Silicon Valley newswire Techcrunch revealed the deal in the early hours of Friday morning UK time, ahead of an announcement expected by the end of this week.
The deal marks the first time two of the leading meta search engines have struck a deal of this size.
Sidestep officials in the US have confirmed the deal but are not disclosing financial details.
Techcrunch suggests financing has been done in two parts: a first round of funding was raised by Kayak worth $27 million followed by a further $197 million – $180 million of which has been used for the cash deal with Sidestep.
Kayak is understood to have around $2.5 billion worth of sales per annum – Sidestep has $1 billion.
Kevin Eyres, who ran the European end of Sidestep, quit in September after 12 months in the post. Annie Wilson currently heads the European wing for Kayak.
The pair have a string of white label deals in the UK including AOL (Kayak) and Orange (Sidestep).