Baroness Thatcher has been invited to open the exhibition celebrating the centenary of the British picture postcard - where the cards shown here will be on display. The invitation (to which she has not yet replied) is not as tongue-in-cheek as might appear. The early Thatcher years - from her appointment as prime minister in 1979 to the miners' strike of 1984 - inspired, or more accurately provoked, the emergence of the political postcard as an important medium of agitprop. Posters were Sixties, lapel badges Seventies, postcards Eighties. They are now highly collectable.

Pete Davies, publisher of three Thatcher postcards, owner of the biggest collection - 400 - and sender of the invitation to open the exhibition, said: 'I hated that woman so much. That's why I started collecting, soon after she became prime minister. Even when she was the education minister I had a real thing about her. That strident voice. That hairstyle. She really got me going. I couldn't get her out of my mind.' During Margaret Thatcher's early days as prime minister, Mr Davies was teaching peace studies and politics in Nottinghamshire schools.

Tony Parkin, a Kent landscape and portrait artist who drew 'Poll Tax - The Final Disaster', published by Mr Davies, and 'Market Forces', voted top political postcard of 1989 by collectors, said: 'She really galvanised me. I only took up postcard design because of her.'

Taken to court for non-payment of poll tax, he paid his debt by selling, for pounds 1 each, Thatcher cards overprinted 'As supplied to bailiffs in payment of poll tax'. He also sold batches of six of his Thatcher postcards in the streets of Edinburgh during the festival last August. 'God, did they hate Maggie in Scotland] One man who bought some cards said when she visited there you could feel the hatred in the air.'

Most familiar image of all is the 1982 spoof film poster of the Socialist Workers Party, 'Gone With the Wind', starring the prime minister and President Reagan. It was the brainwave of a London docker, Bob Light, now doing a PhD in film studies at Warwick University, and John Houston, now a Sunday newspaper design editor, who was art editing for Socialist Worker when Mr Light breezed in with the idea.

The only bit of the original image appropriated (a reproduction of the poster in a book was all that could be found) is the film title. The torsos belong to Linda Quinn, now an official of the National Communications Union, and Mel Norris, SW's managing director at the time, now a typesetter. Mr Norris said, 'I was asked to pose because I was the only one with a proper shirt rather than a T-shirt, boiler suit or sweatshirt.'

As a poster it sold more than 100,000, as a postcard more than 50,000. 'We just couldn't keep up with the printing,' Mr Norris recalled. A copy of it is reported to have been found on the wall of the Argentine army headquarters in Port Stanley in 1982.

In those days, the Socialist Workers Party was an important source of anti- Thatcher printed propaganda. Subversion was in the air. The Labour-controlled GLC was riling the prime minister, and Militant Tendency was tending to be militant. Socialist bookshops selling posters and cards sprang up.

But the 1984 miners' strike took the steam out of agitprop - turning Thatcher postcards into period pieces. Pete Marsden, who was behind the 'Stuff the Jubilee' badges of 1977, and published 'Gone With the Wind' for SW's north London bookshop, said: 'The political climate changed. After the miners' strike a lot of people felt Thatcher had got dug in, that we'd given it our best shot and that was that.'

Besides the SWP, the other big source of anti-Thatcher postcards was the co- operative, Leeds Postcards, which sold mail-order packs of 40 or 50. Richard Honey, one of its directors, said: 'It was given lift-off by Mrs Thatcher's appointment as prime minister. We saw ourselves very much as an agitprop organisation trying to stir up trouble for her and the Tories in general. We loved the idea of doing something we believed in without subsidies or grants.' The modern political postcard, he said, originated in Germany, with a postcard by Klaus Staeck (first seen in Britain in the early Eighties) showing a Rolls-Royce in a back street, captioned 'For wider streets vote Conservative'.

Leeds Postcards is today's most prolific publisher of political postcards, including the Thatcher bestseller from 1982, 'Prevent Street Crime' (35,000 posters, more than 100,000 cards), one of the photomontages which made the reputation of Cath Tate, now an artist- publisher in Brixton, south London. Striking miners adapted it by overprinting 'NUM Food Fund' on the shopping basket of Mrs Thatcher's victim.

'I was flattered,' said Ms Tate. Her delightfully scurrilous book of politico- feminist photomontages, Kissing Toads, came out in 1987, and she now produces feminist cartoon postcards and printed T-shirts - one with the slogan: 'Behind every gifted woman there is a rather talented cat'.

The quest for memorable iconic montages (exemplified by 'Gone With the Wind' and 'Prevent Street Crime') has entered mainstream art. Peter Kennard's Maggie Regina (1983), which appeared in Time magazine, is published by Leeds Postcards as a postcard; his original photomontage of John Major as Mona Lisa (1993) is an artwork that appeared in his exhibition at the Zone Gallery in Newcastle upon Tyne last month. A limited edition of three is for sale at pounds 300 each. The Victoria & Albert Museum has the original of his Constable haywain with cruise missiles (1981).

The most valuable original Thatcher postcard image is Gerald Scarfe's 1983 pen-and-ink 'Nanny Thatcher', bought for pounds 4,510 by the National Gallery at Sotheby's in 1988, making him the most expensive contemporary cartoonist. The gallery shop sells postcards of it for 35p.

As for Thatcher postcards as a collectable, they are easy on the pocket. Out-of- print but run-of-the-mill cards change hands among collectors for pounds 1. Scarcer ones showing significant events such as the Falklands war fetch perhaps pounds 2- pounds 5. Mr Davies has never paid more than pounds 12.50, which bought him the rarely seen Luck and Flaw 'Clouded Tiger' of 1976 with Mrs Thatcher on a donkey.

However inexpensive, as intimations of the 68-year-old former prime minister's mortality hum over the news wires - small faint in Chile, for example - collectors are leafing through their albums ever more lovingly.

'One Hundred Years of British Picture Postcards 1894-1994' exhibition, organised by the Postcard Traders Association, Royal Horticultural Society's Hall, Greycoat Street and Vincent Square, London SW1 (30 August-3 September).

Collect Modern Postcards, 2nd edition compiled by Peter Davies, pounds 5.95 plus pounds 1 p&p, and Postcard Monthly, annual subscription pounds 18, sample copy pounds 1.50 plus 30p p&p, from Brian Lund, 15 Debdale Lane, Keyworth, Nottingham NG12 5HT.

'The Thatcher Years', 10 postcards, pounds 5 inc p&p and catalogue from Leeds Postcards, PO Box 84, Leeds L51 4HU.

Set of six cards by Tony Parkin, pounds 3.50 inc p&p, 47 Oxen Hill Road, Kemsing, Kent TN15 6RG.

Kissing Toads by Cath Tate, pounds 5, five 'Prevent Street Crime' postcards pounds 2, T-shirts pounds 10, all inc p&p from Cath Tate, PO Box 647, London SW2 4JX.

(Photograph omitted)