Factbox - Brexit vote: How many MPs are backing PM May?

LONDON (Reuters) - Britain's parliament is debating whether to support Prime Minister Theresa May's Brexit deal, a decision which will define Britain's departure from the European Union and set the course for the country's economy.

The odds look stacked against May winning parliamentary approval for her deal, which has come under fire from all sides, including eurosceptics, supporters of the EU and the Northern Irish party propping up her minority Conservative government.

Below is a running tally of the number of members of Parliament (MPs) who have indicated during the first of five days of debate whether they will support the Brexit deal on Jan. 15.

MPs are entitled to change their mind before the vote, and some who speak are yet to make a final decision on the deal.

There are a total of 650 MPs. Of those, around 100 are government ministers and Conservative Party enforcers, or whips, who are required to support the deal and so are not included in the tally.

May's Conservative Party has 317 MPs. It governs with a working majority of 13 thanks to a deal with the 10 MPs of the Northern Irish Democratic Unionist Party.

MPs who have indicated they will support the deal: 21

MPs who have indicated they will oppose the deal: 49 (including 10 of May's Conservative MPs)

(Reporting by William James, Elizabeth Piper and Kylie MacLellan; Editing by Catherine Evans)