DBC Foodservice insists it will be able to fulfil its MoD contract after a strike at one of its Scottish depots threatened to disrupt an order destined for HMS Illustrious.

On Thursday, workers at the site in Dundonald, Ayrshire, walked out in a four-day dispute over changes to their terms of employment. DBC was due to dispatch a significant order from the site to the aircraft carrier next Friday and is understood to have shifted production to its Petersfield depot.

It said it had a "robust contingency plan" to nullify the impact of the strike, which centres on a new contract that would increase working hours for Dundonald workers from 37.5 to 45 a week for no extra pay. Union officials claimed that DBC also wanted to abolish the first three days of sick pay.

A company spokeswoman maintained that the "overwhelming majority" of its 1,000 employees had agreed to new terms and conditions, including head office staff, who have agreed to halve lunch breaks to 30 minutes.

However, the proposals were described as "completely unacceptable" by GMB senior officer Mick Conroy. "Asking people to work an additional 7.5 hours for no additional pay is equivalent to a 20% cut in hourly pay rates. These strikes are a last resort for the GMB members who do a vital job of making sure our armed forces have the supplies to do the job they do to protect our nation."

DBC claimed it was just trying to harmonise terms and conditions. "A series of acquisitions prior to 2009 resulted in employees doing similar roles at different sites under varying terms, which has led to inequalities for some employees," said the spokeswoman.

The strike comes amid talks between Purple Foodservice, a consortium of three foodservice providers including DBC, and the MoD to extend its £72m contract to supply 1,200 military ­locations to 2015.

Sources close to DBC said it had battled to keep overheads down as it shifted its focus from the cost to the profit sector.