There are approximately 4.1 million Catholics in England and Wales and many of live in key election battleground constituencies.

The West Midlands, with dozens of crucial swing constituencies, could be where the Catholic vote has its biggest impact.

The Birmingham Diocese has some 286,442 Catholics in the West Midlands area, of which 77,151 attend Mass on a weekly basis. Pope John Paul II has even visited Coventry, landing at the local airport in May 1982.

The West Midlands is a region traditionally regarded as one of the key electoral battlegrounds fought over by Labour and the Conservatives. In the past, it has often exaggerated the national trend, swinging strongly to the Conservatives in 1970 and then to Labour in February 1974.

Another key area is London, which combines the Westminster diocese and the Southwark diocese, which has a combined Catholic population of nearly 850,000. It has also presented an exaggerated version of political trends in the rest of the country over the last 20 years, with the Conservatives doing extremely well in the 1980s and Labour appearing dominant after 1997.

The highest concentration of Catholics in Britain can be found in Liverpool, with over half a million in the diocese. Of these, 70,537 attend Mass on a weekly basis. The city and the surrounding area have a strong, close knit Catholic community, founded on the waves of Irish immigration in the 19th and 20th century.

Liverpool and the North West is often regarded as one of the key barometer regions at general elections, and this time is expected to be no different. The 20 most marginal seats in the area mostly comprise of constituencies where Labour lost to the Conservatives in 1997 and a number saw above-average swings away from Labour in 2001. This could accentuate that trend.

However, the statement by leaders of the Catholic Church is unlikely to spell the end for the most vulnerable Cabinet Ministers in the area: Ruth Kelly. Ms Kelly, who holds Bolton West with a 5,518 majority, is a member of the Conservative Roman Catholic organisation Opus Dei, which should give her added security.

A little north and east, Salford is another Catholic heartland, with 288,026 Catholics, over a quarter of whom attend mass once as week. However this is a more traditional Labour area, with the sitting MP for the constituency of Salford, Hazel Blears, sitting on a 11,012 majority.

Vernon Bogdanor, professor of government at Oxford University, said that despite the match between marginal constituencies and numbers of Catholics, there was only a weak link between religion and voting in Britain.

"It won't carry as much weight as it would have done 40 years ago. If people want to vote Labour over public services or the economy they are unlikely to be deterred by this issue, or not in sufficient numbers.

"On abortion, there will be many people, particularly women, who will be worried about the restriction of choice. The constituencies in Glasgow and Liverpool are so safe so it's not going to make much of a difference."

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