By Julian BirchA bitter inter-union row has erupted on London's prestigious Ludgate development after the site convenor defected to the electricians' union EETPU on Monday.Harry Cowap, who previously represented construction union UCATT, says the EETPU now has majority membership and that more than 150 workers in mainstream construction trades were signed up on Monday alone.But UCATT hit back with a visit to the site by four officials on Tuesday and claimed that workers at site meetings had voted to stay with them.Both sides accused the other of lying and of trying to con site workers with false promises.The Ludgate membership forms were forwarded to the EETPU national executive on Tuesday as evidence of the non-TUC union's biggest breakthrough in the private sector since it set up its construction building trades section in June.The project is a key battleground for the unions in London.Although it is not the biggest site in the capital, its workforce will increase from 400 to up to 1,000 over the next few months.And client Rosehaugh Stanhope and management contractor Bovis are perceived as key players in the debate about the future of industrial relations.The defection of Mr Cowap has not come as a great surprise to close observers of the industrial relations scene. The transport union TGWU had already registered a formal complaint against him because of assistance he gave to the EETPU.Although he is a union convenor he is employed by Bovis, and the firm will be taking a close look at the implications of his defection.His departure, and a similar defection expected within a week on a major site in the north west, were carefully timed by the EETPU to attract publicity.Mr Cowap says he left UCATT because he was 'appalled' and 'sickened' by the treatment of former executive councillors Jack Henry and Jack Rogers and the suspension of general secretary Albert Williams.'Ludgate is now an EETPU site,' he said on Monday.'More than 150 men signed up today. UCATT has no real membership left.'Mr Cowap said the workforces of all the main trade contractors, including concrete frame subbie Swift Structures and brickwork subbie Vogue, had signed up en masse with the EETPU and elected shop stewards.'I can substantiate and prove what we have done. Can UCATT prove their statements if they're challenging what we say?'But UCATT London regional organiser Dominic Hehir hit back with a claim that workers had been 'conned' into signing up with the electricians.'We're confident that we've turned the situation around,' he said. 'This would have been one of the few places the EETPU could have got in and they've failed.'Mr Hehir said more than 100 Swift Structures workers had voted in a meeting on Tuesday to stay with UCATT and elected a UCATT shop steward and safety rep.Mr Cowap responded: 'One hundred men want their dues deducted for us. It's as simple as that.'