The number of banking records the Canadian government is sharing with U.S. tax authorities under a controversial information-sharing deal has increased sharply, CBC News has learned.

The Canada Revenue Agency sent 900,000 financial records belonging to Canadian residents to the Internal Revenue Service in September — nearly a third more than it sent the previous year. The records were for the 2018 tax year.

It also has updated the number of records shared for the 2017 tax year to 700,000 from the 600,000 originally reported.

"That's a lot," said John Richardson, a Toronto lawyer and co-chair of the Alliance for the Defence of Canadian Sovereignty, which is fighting the information-sharing deal. "That's a lot of files."

The number of financial records of Canadian residents being shared with the IRS has risen steadily since the information sharing agreement began — from 150,000 in 2014 to 300,000 in 2015 and 600,000 for the 2016 tax year.

To date, Canada has shipped 2.6 million records of Canadian residents who could be subject to U.S. taxes south of the border.

However, the number of records doesn't necessarily correspond to the number of Canadian residents affected. Some people may have more than one bank account, while some joint accounts could have more than one account holder — including Canadians who don't have U.S. citizenship.

Etienne Biram, spokesperson for the Canadian Revenue Agency (CRA), said the agency does not know why the number of accounts being flagged by Canadian financial institutions is changing from year to year.

"The CRA is currently analyzing the data to gain a better understanding of the fluctuations in the number of records being reported to the CRA."

The information transfer is the result of a controversial information-sharing agreement between Canada and the U.S. that was negotiated after the U.S. government adopted the Foreign Account Tax Compliance Act (FATCA).

The law, adopted in a bid to curb offshore tax evasion, obliges foreign financial institutions to report information about accounts held by people who could be subject to U.S. taxes.

Unlike most countries, the United States levies income taxes based on citizenship rather than residency; some Canadians end up facing U.S. taxes because of an American parent, or because they were born in a hospital on the other side of the border.

One of those Canadians whose banking information could have been shared with the U.S. is Conservative Leader Andrew Scheer, who is a dual citizen of Canada and the U.S.

Toronto lawyer John Richardson says FATCA is prompting some Canadians to renounce their U.S. citizenship. (John Richardson)

Following the adoption of FATCA, the Canadian government concluded that an information-sharing agreement would be better than forcing Canadian financial institutions to deal directly with the IRS.

Under the agreement, Canadian financial institutions send the CRA information on accounts held by clients with U.S. indicia (the fact that the account-holder was born in the United States, for example). Then, once a year, the CRA sends that information to the IRS.

People whose account information is shared with the IRS (names, addresses, account numbers, account balances, interest payments, dividends and other income) are not automatically notified by either their financial institutions or the CRA.

Under the agreement, the IRS is supposed to send the CRA information about U.S. bank accounts held by Canadians. However, the CRA refuses to reveal how many records it has received from the IRS.

"The CRA cannot disclose the number of records received from the IRS under intergovernmental agreement as this is considered treaty-protected information and is subject to the confidentiality provisions of the Canada-U.S. Tax Convention and Section 241 of the Income Tax Act," said Biram

Biram said the CRA is currently examining the way the information is being collected.

"While the CRA monitors the number of records filed each year by Canadian financial institutions, it is still currently developing a compliance program which will allow it to gain a better understanding of this data, including trends and fluctuations in the number of records being reported to the CRA."

Richardson said the number of records being shared with the IRS is likely rising in part because banks and financial institutions didn't initially have to report some kinds of accounts. While the agreement is supposed to apply only to accounts with balances of at least $50,000, Richardson said he believes some institutions are reporting accounts with lower balances.

To date, 2.6 million Canadian banking records have been shared with the Internal Revenue Service. (Susan Walsh/Associated Press)

While Richardson said he hasn't seen any indications the IRS has been taking action based on the information it has received from Canada, it is resulting in some Canadian residents realizing they were expected to file U.S. tax returns.

"There is no doubt that it is pushing a lot of people into U.S. tax compliance," he said. "It's also pushing a lot of people, when they become aware of this, into straight renunciation (of their U.S. citizenship)."

In July, Federal Court of Canada Justice Anne Mactavish ruled the information-sharing agreement does not violate Canada's Charter of Rights and Freedoms.

In September, the group challenging the agreement filed an appeal of that ruling.

Court challenges to bank record-sharing in the wake of FATCA have been launched in the United Kingdom and the European Union.

Elizabeth Thompson can be reached at elizabeth.thompson@cbc.ca