Downing Street will receive a special delivery from postmasters this afternoon, the country's biggest ever petition in modern times. Four million people have backed a campaign, urging the government to save Britain's post offices. More than four thousand post offices have closed since 1999; the rest are losing up to a quarter of a billion pounds a year. Now the National Federation of Sub-Postmasters fears lost income from key services like the planned withdrawal of the Post Office card account in 2010 will threaten the network's future. James Blake reports.

The Saint Levan Valley Post Office near Plymouth faces closure. It is one of thousands that are not commercially viable and cost taxpayers over a hundred and fifty million pounds a year in subsidies. And yet the customers here don't care if the post office makes money, they say it's a social lifeline for the community.

We've had people moved to tears here at the very thought of losing their post office. And we won't fail to express that to the government, we, it's such a vital part of the community.

That's why hundreds of sub-postmasters have descended on London this morning to protest against what they call a death by a thousand cuts. The government has removed so many services from branches that many would be forced to close.

Instead of saying we are going to shut everything, to shut things down, and we're going to deprive communities of their post office, we need, we need to do what we are doing, and that's look at innovative new ways of delivering the service, particularly out in the rural network where there are far less people to be able to generate the income for sub-postmasters to run full time offices.

This afternoon they'll hand in a petition signed by four million people which they hope would preempt any government announcement on cuts.

Now the post office is losing three million pounds a week, and no one I've spoken to here really believes that can go on. This lobby wants reform and modernization, but not the piecemeal destruction of the network they say the government is inflicting on them.

And at the top of the list of grievances: the planned withdrawal of the Post Office card account, used by millions to access pensions and benefits.

There're many, many people who still want, and who budget on being able to get their money from their pension and other benefits each week in cash at post office, and that is the bottom line, the government has given a commitment in the past that would always be able to happen.

They have to be viable .We, what we wanna do is make sure that they can be viable. So in the same way as a pub used to be tied to one brewery, we want to free up, eh, sub-post offices to have lots of products, so they can have a much better income flow, and therefore, stay open.

There are fourteen thousand post offices in Britain; only four thousand are commercially viable. Here in Plymouth, as across the country, they are collecting hearts with messages of support.

But post offices need more than supporters to keep such a huge network alive. They need customers