BRITISH Prime Minister John Major's press secretary, Gus O'Donnell, is to be replaced by a Foreign Office high-flyer. Mr O'Donnell, 41, is returning to the Treasury in January amid pressure from inside the Tory party for the Prime Minister to improve his image. He had been looking to leave for some months and, as a career civil servant, was anxious not to gain an image of ''effectively'' being deputy prime minister, which was attached to Sir Bernard Ingham when he carried out the same role for Margaret Thatcher.. Ministers yesterday denied the move was in response to pressure for a sharper image for Downing Street, but it might have speeded up the process. He will be replaced by the number two at the Washington Embassy, Christopher Meyer, 49, who is used to the media circuit, having been Lord Howe's press man and head of the Foreign Office news department from 1984 to 1989. During that time, which included the signing of the Joint Declaration, he became familiar with Hong Kong issues. Mr Meyer was a popular figure with the press and gained a reputation for calmness under fire, smoothness and affability on the Washington beat. Educated at Cambridge, his Foreign Office postings included Madrid, Brussels and Moscow. He is a complete contrast to Mr O'Donnell, whose south London background is similar to that of Mr Major. Mr O'Donnell went on to Warwick and Oxford universities and lectured at Glasgow University before joining the British Embassy in Washington and then the Treasury. He joined up with Mr Major in 1989 when he was head of information at the Treasury and Mr Major was Chancellor. When Mr Major became Prime Minister Mr O'Donnell was his first appointment. Mr O'Donnell was popular with journalists and removed some of the secrecy of the Downing Street press office. He was very much aware of the way Sir Bernard's extremely close association with Lady Thatcher effectively prevented him from returning to the civil service. There were suggestions that he wanted to get out of Number 10 before his own career became tarnished with the failures of the Major administration. By appointing Mr Meyer, the Prime Minister has signalled that he does not intend to submit to the demands of some of his advisers for an outside senior public relations man to be hired to bolster his image.