Fasset, which operates Langstone Technology Park at Havant, has embarked on the 10-week refit to accommodate more people after reaching a record 99 per cent occupancy level.

The largest manufacturing cleanroom of its kind in the world is being demolished to make way for offices, a warehouse, a data processing centre and a call centre.

The 43-acre park has 3,000 people working for 18 companies including defence company Lockheed Martin, electronics group Hitachi and telecoms specialist AT&T.

Fasset, which creates and manages business space on behalf of corporate occupiers and property investors at similar sites across the south, expects to complete the development later this month.

A further 60,000 sq ft around the cleanroom is also being transformed.

The first refit phase at the building – named 5000 - has seen a call centre built within just eight weeks for the UK's third largest energy supplier.

Scottish and Southern Energy plc (SSE) signed up 20,000 sq ft, with room for up to 220 employees, for an undisclosed rental figure.

Gary Medlow, Fasset's joint managing director, said: 'Building 5000 represented a huge chunk of under-utilised, expensive surplus space over 110,000 sq ft.

'We're transforming it into offices, a warehouse, a data processing and a call centre in order to make it pay its way.

'LTP is full to bursting now, with 99 per cent occupancy, so the scheme will create opportunities to accommodate at least 500 additional people, as well as meeting the needs of clients.

'We're creating new space for the market place, broken down into manageable sizes and available on competitive, short-term leases.

'Also, what had been an unsuitable space is being turned into a driver for regional economic development, which is one of our guiding lights.'

LTP had an 80 per cent vacancy at the turn of the millennium.

But Fasset turned its fortunes around with a rolling £18m investment and the freehold was sold to a property investment group for £54.5m nearly three years ago – more than double the value in 2000.