The ANC plans to ask the Constitutional Court to nullify the results of 12 of Wednesday's Western Cape by-elections.

"The reason we are asking the Constitutional Court to intervene in this matter is because the ANC's exclusion from contesting the 12 by-elections disenfranchises tens of thousands of voters," provincial chairperson Mcebisi Skwatsha said on Thursday.

All 12 ? eight in the City of Cape Town and four in the Cedarberg ? were for seats formerly held by the ANC.

The Electoral Court this week upheld the Independent Electoral Commission's decision to bar the party's candidates for not meeting a registration deadline.

Skwatsha said the party respected the rules governing elections and recognised the principle of equality before the law.

"However, for the ANC to be prevented from contesting by-elections ? in wards previously held by the party ? distorts the political landscape," he said.

"It creates conditions for councillors to be elected with little community support.

"This scenario will not instil community confidence in local government and is likely to have a negative effect on service delivery."

Earlier on Thursday, the Democratic Alliance said the Western Cape results showed the party was on track to win the province in next year's general election.

DA chief executive Ryan Coetzee said the party had won nine of the 18 wards in the province previously held by the ANC.

Four of these wards had been contested by the ANC, which currently has a majority in the provincial legislature.

"There's one simple message from yesterday's by-elections in the Western Cape... that the DA is on track to win this province, if not with an outright majority, then certainly on track to be the biggest party in the Western Cape by quite some distance," Coetzee said.

There had been a "quite spectacular" swing away from the Independent Democrats to the DA in Cape Town, and significant increases in DA support in black wards.

Coetzee said the results showed that Cope was splitting the ANC vote, and not eating into the DA's support base.