Lawyers for French engineering giant Alstom have written to the GMB union threatening legal action if it goes ahead with planned strikes in September

The letter was issued on Monday as the union prepared to send out ballot papers to members involved in the engineering construction sector.

More than 140 GMB and Unite shop stewards voted unanimously in June in favour of industrial action after pay negotiations broke down with the Engineering Construction Industry Association.

The letter threatens a legal challenge should the strikes go ahead. It says: 鈥淎lstom reserves the right to seek an interim injunction to prevent this industrial action and/or sue the GMB for damages.鈥 It also says it believes it would be within its rights to sack striking workers.

Phil Davies, GMB national officer, said the firm had questioned how far the strikes were trade disputes, because one point of contention concerned the employment of foreign workers.

鈥淭he strike isn鈥檛 about foreign workers, it鈥檚 about tightening the audits to ensure workers get fair rates of pay and conditions, and reaching a settlement for 2010 pay terms,鈥 Davies said.

We expect all sorts of reasons why we shouldn鈥檛 ballot. But we will go ahead

Bob Blackman, Unite

He added that he was pressing ahead with the ballot.

The ballot will run until 1 September, with industrial action expected to follow shortly after.

Seven sites will be targeted by the unions, including Alstom鈥檚 sites at Staythorpe and Sellafield.

Bob Blackman, the national secretary of Unite, said: 鈥淲e expect all sorts of frivolous reasons why we shouldn鈥檛 ballot,鈥 he said. 鈥淏ut we will go ahead with what we have to do.鈥

An Alstom spokesperson denied the letter was a threat of action and said: 鈥淲e鈥檝e just asked the GMB to explain why it thinks it has a trade dispute with Alstom.鈥