Monsanto, the agrochemical and seed giant, is eyeing a move to the UK as part of a complex merger with Swiss rival Syngenta that would take advantage of Chancellor George Osborne's lower corporation tax rate.

The US firm is proposing a so-called "corporate inversion," or "tax inversion," which would involve setting up a new company registered in the UK.

Syngenta rejected the $45 billion revised bid from Monsanto on Monday, priced at $489.84 per share – a 43 percent premium on current prices.

The US agrochemical firm, known for its genetically modified (GM) crops, also offered a $2 billion termination fee to be paid in the event an anti-trust regulator blocks the merger.

Under the plan, Monsanto investors would end up with a 70 percent holding in the UK company while investors in Syngenta would take the rest.

Syngenta shareholders would also receive cash, which when combined with shares in the new firm would reach a total value of around $45 billion.

"A new parent company, domiciled in the UK, would demonstrate that our merger will create a new global enterprise … [and] provide additional synergies," Monsanto CEO Hugh Grant wrote.

"We would also propose a new name for the combined company to reflect its unique global nature."

The Swiss firm rejected the offer, saying it included "the same inadequate price, same inadequate regulatory undertakings to close, same regulatory risks."

Britain has become a more attractive destination for corporations to set up their headquarters since Chancellor George Osborne cut corporation tax from 21 percent to 20 percent and established generous rules on the taxation of complex overseas subsidiaries based in the UK.