British Transport Police need an extra 200 staff across the UK to cope with the increased threat of terrorism, according to a senior union official.
Police patrols have increased on the London Underground
British Transport Police federation chairman Alex Robertson said funding cuts would mean 50 fewer officers.
He was speaking as security was stepped up on the London Underground to help deter a terrorist attack.
A Transport Police spokesman admitted the 2004/05 budget had left the service facing "a real challenge".
Extra patrols, using plain clothes and uniformed officers, are being carried out at Tube stations in London. A poster campaign has also begun on the underground.
Mr Robertson told the BBC that the staffing issue was most acute for the London transport network, but that it was also a problem for the rest of the UK.
The BTP spokesman said that while the 2004/2005 budget settlement had increased by 20% on the previous year to £125m, expenditure had also risen.
A £3.5m VAT bill and a £10m deficit in the service's pension scheme had put pressure on funding.
At present the service is paid for by the 27 train operating companies but Mr Robertson said the UK Government should take responsibility.
He said: "For the last four years this federation has told the government that we have had a lack of funding to such an extent that this year's budgetary requirements have left us 50 men short for the future.
The Madrid bombings served to increase the terrorism threat in the UK
"I don't think in the climate we are in at the moment with the highest terrorism threat in the UK, we should be losing manpower.
"It should be the exact opposite, we should be increasing manpower by at least 10% which would give us an extra 200 officers to deal with this threat.
"Everywhere in the UK is a threat, but intelligence tells us London is the major threat. Nowhere is safe on the transport network in the UK."
The service could probably cope at present but he was worried that by moving around resources to deal with the threat of terrorism, other BTP duties would be neglected.
He said: "Long term, we cannot retain the level of high-profile policing that will be needed in the coming months after the Madrid atrocity.
"The terrorism threat will be a main part of our business, but if we do that other things will slip because we will obviously have to move officers to that in the long term."
Mr Robertson said it was important to the public to see police in high-profile places like tube and rail stations.
The BTP spokesman said that patrols on London's Underground had been increased but he admitted that the overground rail network might lose out.
"We will do what we have to do. Counter-terrorism is at the top of our agenda, but we are also committed to robbery patrols and football policing.
But he said areas such as anti-vandalism might suffer because "you cannot do everything".
The spokesman added that the service would be more intelligent about how it ran policing on the network and it would continue to reduce such things as sickness in the service.