BUDAPEST (Reuters) - Hungary stepped up its anti-immigration measures on Thursday with plans to introduce a 25 percent special tax on aid groups which support migration.

Prime Minister Viktor Orban's government has been among the most hostile to immigration in Europe and was re-elected in a landslide victory in April on a pledge to crack down on NGOs it says undermines Europe's integrity by supporting migrants.

"We want to use tax policy to step up against organising migration," Hungary's Finance Ministry said in a statement.

After its election victory, Orban's government swiftly submitted a bill to Parliament aimed at criminalising aid to migrants, threatening activists with jail terms.

The original version of the bill, submitted in February, included a 25 percent tax levied on NGOs whose funding comes mostly from foreign donors, but that clause was dropped.

Now the finance ministry says it will reintroduce the proposed tax in a separate piece of legislation.

The ministry said the "immigration special tax" was necessary because defending against illegal immigration put a significant extra burden on the budget.

It was not immediately clear what the new tax would be levied on, nor when it would first be collected.

(Reporting by Marton Dunai; Editing by Andrew Heavens and Alexander Smith)