U.S. investment firm Long Path Partners said Friday that it has so far won backing from 19.12% of shareholders in Idox PLC for its £339.5 million ($461 million) buyout of the U.K. government software company.

Long Path said that the backing includes irrevocable undertakings from the directors of Idox and other shareholders that hold a total of 15.63% of the software company's stock.

The U.S. investment firm already owns 12.32% of Idox.

The offer has been open to Idox shareholders for 21 days. The investors have until March 16 to tender their shares under the offer.

The deal is being completed as a takeover offer and is subject to Long Path holding more than 50% of shares in Idox. Long Path said it will be able to squeeze out any remaining shares if it holds at least 90% of Idox's shares.

Long Path said it will pull Idox's shares from trading on London's Alternative Investment Market if it holds at least 75% of the stock.

Long Path, through its bidding vehicle Frankel UK Bidco Ltd., is offering Idox shareholders 71.5 pence in cash per share. The U.S. company said in October that it had agreed to buy Idox in a deal that requires U.K. and French foreign direct investment clearance.

The buyer had previously intended to complete the deal through a scheme of arrangement, which requires backing from at least 75% of shareholders in the target. But it changed this to a takeover offer in January to make it easier for the deal to go ahead.

Idox, which is based in Surrey, southeast England, provides software to help manage elections.

Kirkland & Ellis LLP is guiding Long Path, with a team led by corporate partners Stuart Boyd and Victoria Legg. Pinsent Masons LLP is steering Idox with a team headed by partner Rosalie Chadwick and public mergers and acquisition specialist Adam Cain.