UPDATE 2-FTSE 100 gains as pound hit by Brexit deal legal opinion

Britain's blue-chip index gained on Tuesday after a fall in the pound triggered by the government's lawyer saying last-minute assurances won by Prime Minister Theresa May for her Brexit deal left the risk over the so-called Irish backstop "unchanged".

The export-heavy FTSE 100 was up 0.1 percent by 1329 GMT, while the more domestically-focussed FTSE 250 was up 0.2 percent. Ireland's main index, seen as a gauge of Brexit jitters, rose 0.7 percent. Domestic banks and housebuilders had gained ground on relief Britain may avoid a disorderly exit from the European Union after won "legally binding" Brexit assurances from Brussels.

Tuesday's FTSE 100 turnaround began after official data showed Britain's economy picked up in January after a weak December but growth was still stuck in low gear ahead of Brexit. The weakness in the pound following Attorney General Geoffrey Cox's comments spurred blue-chips further.

Banks with a big domestic focus and housebuilders, considered to be vulnerable to economic hits stemming from Brexit, were among the best performers on the index. Barclays added 2.3 percent, state-owned lender Royal Bank of Scotland was up 2.8 percent and homebuilders Persimmon and Taylor Wimpey added 1.7 percent and 1.5 percent respectively.

British lawmakers, who on Jan. 15 voted 432-202 against May's deal, were studying the assurances and Cox's legal advice before the vote later on Tuesday. It is not certain May will win the vote, leaving much still hanging in the balance and investors nervous about the outcome.

"I expect sterling to come under further pressure in the short term unless May is able to get her Brexit deal approved in parliament, something I now give a 10 percent chance," JCI Capital portfolio manager Alessandro Balsotti said. Spreadex analyst Connor Campbell suggested that though the expectation remained that May would lose the vote, the size of the loss could now be far less than it would have been.

Mid-caps saw some news-related moves. Among gainers, Sirius Minerals added 8.8 percent after announcing an alternate financing proposal with lenders while wealth manager Quilter advanced 6.3 percent after better-than-expected results.

Administration and payment service provider Equiniti fell 8.3 percent after a delay in the separation of its U.S. business from Wells Fargo and lower 2018 pretax profit. Oil and gas company Cairn Energy slipped 2.8 percent after reporting lower-than-expected output from Kraken field.

High street retailer Mothercare was set for its best day since early October after detailing plans to sell its educational toy brand Early Learning Centre as it seeks to cut debt. The stock was up 7 percent. (Reporting by Shashwat Awasthi and Muvija M in Bengaluru, additional reporting by Danilo Masoni; editing by Josephine Mason/Keith Weir/Alexander Smith)