Celtic have no intention of letting 'keeper Fraser Forster leave the club and have gone to the extent of formally writing to a European rival - believed to be Benfica - to warn them off. England manager Roy Hodgson has called Forster up to his squad for the World Cup qualifiers with Moldova and Ukraine which will do no harm to the goalkeeper's value. Forster has already been linked with a move to the Stadium of Light in Lisbon, but manager Neil Lennon sees the former Newcastle 'keeper as integral to his plans for the club's Champions League games with AC Milan, Barcelona and Ajax. Forster was a key player in last year's run to the last 16 in the competition and Lennon insists Foster is going nowhere. He said: "Fraser's not for sale. We had no confirmed bids for him. We have actually written a letter to one club, just warning them off basically."Yesterday Celtic signed Nir Biton from FC Ashdod for a fee of around £1 million. Lennon hopes to be able to confirm another signing today with £3 million heading to Schalke in return for striker Teemu Pukki. Celtic still have a gap in the left-back position that Lennon would like to fill. If he manages to find somebody before midnight tonight it would take the number of summer signings to seven after the captures of Amido Balde, Virgil van Dijk, Derk Boerrigter and Steven Mouyokolo.However, Lennon admits adding to his squad has been a frustrating process. He said: "We've been looking everywhere, all over the world, really - South America, Canada, places like that and sometimes it's very difficult to do deals. Sometimes you think you're there and then the price goes up, another agent gets involved and you end up talking to three different agents. It just becomes a bit of a muddle. We do our business cleanly, simply and quickly, but sometimes that can't be the case. I think there's money there to spend, it's just about trying to get the right player in for the right price."