Spirent, the Sussex-based telecoms company, has entered the list of the UK's biggest companies.

Formerly known as Bowthorpe, it has replaced the likes of Rolls-Royce and Hanson in the FTSE 100.

Market capitalisation for the Crawley firm is in the region of £3.5 billion and growing.

Most of the company's operations are in the U.S.

Chief executive Nicholas Brookes said there were plans for Spirent to register on New York's NASDAQ exchange to make it easier for American investors to take a stake in the company.

It had been the rapid growth in the market for telecoms-testing equipment that had driven the growth at the electronics group.

Spirent said its telecoms-testing equipment division, Spirent Communications, had seen turnover and operating profit grow by 80 per cent in the past six months.

The communications division contributed 66 per cent of operating profit during the six months to June 30, compared with 37 per cent at the same point last year.

The firm employs more than 7,000 people although only 70 are based at the state-of-the-art world headquarters building in Crawley.

Turnover in the six months to June 30 rose to £316.4 million, from £251.7 million.

But an £11 million loss incurred on the disposal of three of its smaller business impacted on pre-tax profits during the period, which came in at £32.7 million against £38.5 million at the same time before.

Mr Brookes said the first six months had seen an "outstanding performance" by Spirent Communications, which, with increasing investment being made by the telecommunications industry, meant the company was well placed for the second half of the year.

Mr Brookes said that unlike many fast-growing companies, Spirent had a solid base.

"We now have a focused company concentrating on the telecoms business. We make real profits and we have a strong background.

"We intend to keep growing our business in the UK and in the U.S.

"Last year we invested record amounts in research and development, which is where much of our future growth will be coming from."

He added the company had not suffered excessively from the effects of the weak euro because 60 per cent of business was done in the U.S.