UPDATE 1-Qinetiq faces strike threat from over 2,000 staff

* A third of the workforce to vote on strike action

* Walk-outs could hit Chinook helicopter project

(Adds Union, company comment, detail)

LONDON, May 18 (Reuters) - Thousands of staff at British defence technology firm QinetiQ (QQ.L) are to vote on potential strike action in a dispute over pay.

Scientist trade union Prospect said on Monday it and three other unions would ballot up to half the 7,000-strong work-force at QinetiQ -- once a division of Britain's Ministry of Defence that tested the country's weapons.

"We are balloting members for a combination of strike action and a ban on working outside normal hours," Prospect's National Secretary David Luxton told Reuters, adding the dispute stemmed from a company decision to freeze pay for the majority of workers this year.

He said possible strikes, set to take place next month, could affect upgrade work on MoD Chinook helicopters.

QinetiQ disputed the number of workers likely to be involved in the vote, saying around a third of its employees were union members against Prospect's estimate of a half.

A QinetiQ spokesman said the company had frozen pay to save cash during the economic downturn. "It's very disappointing. We are committed to continuing constructive discussions on pay," he said.

QinetiQ chief executive Graham Love told staff in April he too would not take a pay increase, but Prospect's Luxton said Love would still be taking a bonus. QinetiQ's short lifespan as a private company has been dogged by controversy. The government first sold a stake to U.S. private equity group Carlyle in 2003, but suffered heavy criticism for selling too cheaply after the group's 2006 flotation saw investors reap massive profits.

QinetiQ said in February it would look to review its cost base due to belt tightening by the British government, although it also has operations in North America.

It was due to report results for the year to end-March on Thursday. Its shares, down 9 percent this year, closed up 1 percent at 145.25 pence, valuing the business at 957 million pounds ($1.46 billion). (Reporting by John Bowker and Myles Neligan; Editing by Anshuman Daga and Dan Lalor) ($1 = 0.6556 pound)