The Wales Office is set to see its annual budget almost double from £4.68m to £8.44m.
The Wales Office will see its budget rise by £3.76m next year
Opposition parties say they are concerned at the level of increase.
But the Wales Office says the new money is for the expected increase in work dealing with the extra law-making powers handed to the Welsh assembly.
Welsh Secretary Peter Hain said the budget would fall again in a few years, but said he would "make no apology for having a strong policy base".
"While I accept there is work to be done at Westminster handling the assembly's bids for legislation, that cannot equate to a doubling of the Wales Office budget in a year," said Conservative finance spokeswoman Angela Burns.
"Front line public services are being squeezed with tiny increases in expenditure over the next three years, yet the Wales Office appears to have escaped."
The £3.76m increase for next year is a jump of just over 80%. In 2009, the budget will stay at £8.25m, before dropping to £4.4m in 2010.
A Wales Office spokesperson said: "The settlement reflects the real running costs of the Wales Office until 2010-11, in light of the increased workloads of managing legislative opportunities under the Government of Wales Act 2006."
It said the assembly government had been kept fully informed on the budget agreement, which is funded directly from the Treasury.
Mr Hain told BBC Radio Wales it was "actually a pretty flat settlement" and the Wales Office had been using up its reserves in the past few years when it had a budget of "just over £5m".
The Wales Office was set up in 1999 after most of the powers of the old Welsh Office were transferred to Cardiff Bay.
Mr Hain and his ministers are now expected to liaise with the Welsh Assembly Government, AMs and the UK government on any proposals for new laws coming out of Wales.
'Value for money'
That work is already underway, after the first proposal to devolve extra powers on education to Cardiff Bay were discussed by the Welsh affairs committee in the House of Commons this week.
Liberal Democrat Jenny Randerson said: "I'm shocked that so much additional money is going to the Wales Office when it's such a tough settlement for everyone else.
"Council tax payers are facing a hike in bills, yet Peter Hain's department is getting a massive increase in its budget."
Plaid Cymru MP Adam Price said: "It's staggering and it's hard to fathom what all these extra people are going to do.
"It's not the Wales Office, it's the waste office: £8m - what does that buy you?
"A new hospital, community health centres. You have to ask if we're getting value for money from the Wales Office."
The assembly government has declined to comment.
But Alun Ffred Jones of Plaid Cymru - which is coalition partner with Labour in the assembly government - said he would be looking for clarification on how the Wales Office money will be spent.
"At a time when the budget is so tight [in Wales], we need to ensure that value for money is achieved across all government departments," said Mr Jones.