The Ministry of Defence is looking to appoint what could be the country's best-paid civil servant on a salary of up to £500,000, a move that unions have blasted as "utterly outrageous".

The Government will pay the next chief executive of Defence Equipment & Support, the £14bn-budget agency that buys the UK's military kit, a £250,000 salary plus a bonus of up to a further £250,000, according to the recruitment advertisement.

Data released by the Cabinet Office this year showed that the best-paid public officials were on up to £330,000 a year – David Cameron earns £142,500. The current DE&S chief executive, Bernard Grey, who is paid £220,000, has overhauled the organisation and is expected to reapply for the job, which involves trying to ensure the MoD buys better equipment at a lower cost.

A spokesman for the Public and Commercial Services Union said: "This is typical, utterly outrageous – and an insult to hard-working public-sector workers."

Kevan Jones, Labour's defence spokesman, said: "Thousands of members of the armed forces have been made redundant and all service personnel have seen a deterioration in their pay and conditions. Ministers must justify their reasons for paying such an extraordinarily high salary to one civil servant."

But a MoD spokesman insisted: "This is a key role responsible for... £14bn of spending on the most complex procurement in the world... Any bonus will only be paid if the incoming chief executive secures a good deal."